I'm pretty new to perl as you can probably tell from my solution, but here you go anyway.

#!/usr/bin/perl -w

use strict;

my $someDatFile = 'datfile.dat';    #the file you need read in

open( DATFILE, $someDatFile ) or die "Cannot open $someDatFile";

my $currentPrefix;
while( <DATFILE> )
{
    my( $prefix, $postfix ) = split ( /,/, $_ );
    &cleanUp( \$prefix );
    &cleanUp( \$postfix );

    if( ( !$currentPrefix ) or ( $currentPrefix ne $prefix ) )
    {
        if( $currentPrefix )
        {
            print "\n";
        }
        $currentPrefix = $prefix;
        print "$prefix, $postfix";
    }
    else
    {
        print ", $postfix";
    }
}

print "\n";

close( DATFILE );

sub cleanUp
{
    my $ref = shift;
    $$ref =~ s/^\s*//;
    $$ref =~ s/\s*$//;
}
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