Just off the top of my head, could you not do something like this (untested code):

#!/usr/bin/perl -w
use strict;

my $infilename  = 'blah.dat';
my $outfilename = 'blah.out';
my (%data);

open(DF, $infilename) 
  or die("Can't open $infilename for input: $!\n");
while (my $line = <DF>) {
    chomp($line);
    my @parts = split(/\s/, $line, 2);
    push(@{$data{$parts[0]}}, $parts[1]);
}
close(DF);

open(OUTF, '>' . $outfilename) 
  or die("Can't open $outfile for output: $!\n");
foreach my $k (sort(keys(%data))) {
    print OUTF "<p>", join(', ', @{$data{$k}}), "</p>\n";
}
close(OUTF);
[download]

Basically, the idea would be to push the data into an array, then join them as you liked.

Hope that helps....