I used to do exactly the same as come of the comments here suggested: print statements, data dumper, and so on

Now I'm going 'agile' by including unit tests from the beginning of my coding, even if inheriting code from others.

In fact, I write tests first, then start coding to make them work. It's a different approach. My 'debugging' time decreased in the end because I was able to find many more mistakes early on.

Give it a try, google on unit testing. There's even a book on Extreme Perl and Test-Driver Design

An example: a simple accumulator. I start with writing Add.t first (the code at the end) and fill up Add.pm (first code on top) to make Add.t run without failures...

A simple accumulator

package Add;

use strict;
use warnings;

sub new {
    my $class = shift;
    my $self = {
        ACC => undef,
    };
    my $closure = sub {
        my $field = shift;
        if (@_) { $self->{$field} = shift; }
        return $self->{$field};
    };
    bless ($closure,$class);
    return $closure;
}

sub acc { &{ $_[0] }("ACC", @_[1 .. $#_]) }

sub add {
    acc( $_[0],acc( $_[0] )+@_[1 .. $#_] );
}

1;
[download]

The test code

use strict;
use warnings;

use Test::More tests => 7;

BEGIN {
    use_ok('Add');
}

ok(my $a = Add->new(),"creation of accumulator");
ok($a->acc(3),"accumulator set to 3");
isnt($a->acc,0,"check if accumulator isn't zero");
is($a->acc,3,"check accumulator equals 3");
ok($a->add(3),"add 3 to accumulator");
is($a->acc,6,"check if add equals 6");
[download]