Why does substr work differently when passed directly into a method? - arrays

Question

I've a question about perl that I used to not bother about in the past, but it's bugging me now.
I have a method call saveItems which takes in a value from a text log and parses the input.
so I have this few lines in the method.
$intime = $_[1];
$timeHr = substr($intime, 0,2);
$timeMin = substr($intime, 2,2);
$timeSec = substr($intime, 5,2);
$object[$_[0]]->hr($timeHr);
$object[$_[0]]->min($timeMin);
$object[$_[0]]->sec($timeSec);
$intime being the value of the time passed into this method.
Sample of $intime: 0431:12
My question is that why does the above not give me any error but when I try to shorten the lines like so :
$object[$_[0]]->hr(substr($intime, 0,2));
$object[$_[0]]->min(substr($intime, 2,2));
$object[$_[0]]->sec(substr($intime, 5,2));
Only the first one works while the rest gives me an out of string error.
I am relatively new to perl, as you can see, but can anyone give me an answer to this?
EDIT
Sample HR:
sub hr {
my $self = shift;
if (#_) { $self->{HR} = shift }
return $self->{HR};
}
EDIT
Case Closed.. Read my answer post

Answer 1

From the comments above, adding .'' after each substr solved your problem. The reason for this is that the ->hr, ->min, and ->sec methods are modifying their argument in some way. Without seeing it further I can't say for certain what is happening.
The substr function returns a value that is a valid lvalue. This means that it can be assigned to. So when something in those methods assigns to the slice from substr, it is interfering with the other methods.
Appending an empty string fixes the problem by breaking the alias between the slice and the original string (stored in $intime).
If you wrote the hr, min and sec methods, you should figure out why they are modifying their arguments. Adding print "[$intime]\n"; statements between each method call should be revealing.

Answer 2

Can you come up with self-contained runnable code that demonstrates the problem? The problem you describe doesn't quite match up with the code you show, though I don't understand #object's role in your code.
The following works just fine:
use strict;
use warnings;
package Class;
sub new { bless {} }
sub saveItems {
my $intime = $_[1];
$_[0]->hr(substr($intime, 0,2));
$_[0]->min(substr($intime, 2,2));
$_[0]->sec(substr($intime, 5,2));
}
sub hr {
my $self = shift;
if (#_) { $self->{HR} = shift }
return $self->{HR};
}
sub min {
my $self = shift;
if (#_) { $self->{MIN} = shift }
return $self->{MIN};
}
sub sec {
my $self = shift;
if (#_) { $self->{SEC} = shift }
return $self->{SEC};
}
package main;
my $object = Class->new();
$object->saveItems( '0431:12' );
print "hr: ", $object->hr(), " min: ", $object->min(), " sec: ", $object->sec(), "\n";

Answer 3

This matter has been resolved.
The way of using substr as follows, are able to perform normally, without errors.
$object[$_[0]]->hr(substr($intime, 0,2));
$object[$_[0]]->min(substr($intime, 2,2));
$object[$_[0]]->sec(substr($intime, 5,2));
However, it is the log file that has trailing blank lines that got this script to fail.
Thanks to #ysth for asking me to reproduce the problem, when I realized that the problem actually lies with the log file instead of the script.
Lesson learnt: Check the codes AND the source before raising an issue