When you run $PairsOf{$Person1}        = $PairsOf{$Person1} . " " . $Person2;, then the second occurence of $PairsOf{$Person1} isn't defined yet, which is why you get the warning.

I'm not sure what you want to achieve, so I can't help you to fix it.

Quick fix:

my $Person1 = defined($1) ? $1 : '';
my $Person2 = defined($2) ? $2 : '';
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But that's just covering the problem, assuming your data always has two persons in each 'Pairing Start' line. A better solution would be to re-write the regex in line 17: It's very greedy, and is probably grabbing more than you intend in several places.

First off, drop the last '.*'. You aren't saving it, and you haven't anchored the regex on that end, so it's not doing anything except eat processor cycles. (Besides, anything it could have matched is already caught in the last capture.) Next, define exactly what is allowed in a 'Person', and refine your two captures to just capture that.

What I suspect is happening is that some of your lines have whitespace at the end. This will then mean that the whole line ends up in $Person1, and $Person2 is empty. (Because the whitespace requirement is the only thing stopping the first capture from capturing the whole rest of the line.) I'm guessing putting (\w+) as your captures will likely do what you want, and solve your problem.

What I am trying to achieve is a set of Pairs which is then (in the script that this code fits in) fed into another script which performs some functions on each pairs defined.

I tried:

my $Person1 = defined($1) ? $1 : '';
my $Person2 = defined($2) ? $2 : '';
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And the end results were the same.

So then I tried re-writing the regex to this:

if ($Line =~ m/^Pairing Start\s+(\w+)\s+(\w+)/) {
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And I go the same results:

Use of uninitialized value within %PairsOf in concatenation (.) or str
+ing at ./test_removing_pair_unitialized_value_error.pl line 26, <DATA
+> line 14.
Use of uninitialized value within %PairsOf in concatenation (.) or str
+ing at ./test_removing_pair_unitialized_value_error.pl line 27, <DATA
+> line 14.
Use of uninitialized value within %PairsOf in concatenation (.) or str
+ing at ./test_removing_pair_unitialized_value_error.pl line 27, <DATA
+> line 14.
Use of uninitialized value within %PairsOf in concatenation (.) or str
+ing at ./test_removing_pair_unitialized_value_error.pl line 27, <DATA
+> line 14.
Use of uninitialized value within %PairsOf in concatenation (.) or str
+ing at ./test_removing_pair_unitialized_value_error.pl line 27, <DATA
+> line 14.
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I also checked the __DATA__ to ensure there were no white spaces at the end of each and every line and I wasn't able to find any..

Any other Idea's?

I tried: ... And the end results were the same.

$Person1, $Person2 are not the problem

The problem is %Pairs and %PairsOf

$PairsOf{$Person1} is not defined (it is uninitialized), but you assign to it the value of $PairsOf{$Person1} (an uninitialized value)

Its like writing

my $foo = undef;
my $bar = 'bar';
$foo = $foo . $bar; # UNITIALIZED!!!
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Get it?

You are confirming what I suspected...And I understand what you are saying... The only problem is that I tried your code and it doesn't seem to give the expected result

#!/usr/bin/perl

use warnings;
use strict;

my $foo = undef;
my $bar = 'bar';
$foo = $foo . $bar; # UNITIALIZED!!!

print "foo = <$foo>\n";
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prints: foo = <bar>
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So thanks for the help though, It still help's me understand what is happening.

In Perl >5.10, there is a new operator //=. This checks for "definedness". $Pairs{$Person1}   //= $Person1; the hash value is assigned the value of $Person1 if that hash value wasn't already defined.

You cannot use an undefined value in a string concatenation. So, $PairsOf{$Person1} //= ""; will define $PairsOf{$Person1} to be a null string if it isn't already defined. If the value is already defined, then this should have no effect. A null string can be used in a concatenation.

      $Pairs{$Person1}   //= $Person1;
      $Pairs{$Person2}   //= $Person2;
      $PairsOf{$Person1} //= "";
      $PairsOf{$Person2} //= "";
      $PairsOf{$Person1}  = $PairsOf{$Person1} . " " . $Person2;
      $PairsOf{$Person2}  = $PairsOf{$Person2} . " " . $Person1;
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Now this fixed my problem, thank you very much!, I now have more homework to do on learning the new features of perl :)

thanks again!