Thank you all for taking the time to help me out! It's truly amazing how many ways there are to do something ;) I've learned a lot by reading your replies and I've cleaned up the code a little and tweaked it with the input I got from each of you. here it goes;

use strict;
use warnings;
use File::Basename;
use Text::ParseWords;

if ($#ARGV == 0) {

        open my $file, "<", $ARGV[0] or die "Couldn't open file '$ARGV
+[0]': $! \nDid you specify a valid file?";

                my ($ID,$ISIN,$SYMBOL);
                while (<$file>) {
                if ($_ =~ m/ID:/) { $ID = getValue($_); }
                if ($_ =~ m/Symbol:/) { $SYMBOL = getValue($_); }
                if ($_ =~ m/ISIN:/) {
                        $ISIN = getValue($_);
                        print "$ID:$ISIN:$SYMBOL\n";
                        }
                }

} else {
        print "You need to specify an input file \n";
        print "Usage : ".basename($0)." difffile.txt \n";
        exit;
}

sub getValue {
        $_ =~ s/\s+//g;
        my ($name, $value) = split(/:/);
        chomp($value);
        return $value;
        }
[download]

please let me know if there are any more improvements that can be done to make it more "clean" and "proper" according to "proper coding standards" ;9 Thank you all !!

Output from diff_file, running :
./isin_parse.pm diff_20120614T1442.txt

QYP:SE0001234567:LUP2L80OHM
NGA:SE0001234567:SHB2K260OHM
NJQ:SE0001234567:TRE2K70OHM
NH0:SE0001234567:SKF3A190OHM
NEY:SE0001234567:NOK3A70OHM
QUA:SE0001234567:MINILONGLUPFO
QPC:SE0001234567:MINISHRTOMXOO
QP3:SE0001234567:MINISHRTOMXFO
QU9:SE0001234567:MINILONGLUPEO
P3B:SE0001234567:SWM2K240OHM
P0N:SE0001234567:ASS3A160OHM
NGF:SE0001234567:SHB3A260OHM
R1B:SE0001234567:SKF2K210OHM
QP8:SE0001234567:MINISHRTOMXKO
R1K:SE0001234567:TEL3A123OHM
P34:SE0001234567:SWE2K140OHM
NAS:SE0001234567:ASS2K180OHM
NC3:SE0001234567:BOL3A150OHM
NDD:SE0001234567:HM2L280OHM
NCX:SE0001234567:ERI3A120OHM
P2D:SE0001234567:OXS3A1300OHM
QNA:SE0001234567:MINILONGABBCO
NGV:SE0001234567:SKF2K190OHM
QPE:SE0001234567:MINISHRTOMXQO
QU7:SE0001234567:MINILONGLUPCO
[download]

nt!!