If you want to stay with cat-ing the file then just do this

 
use strict;
my $firstfile;
my $secondfile;

$firstfile = `/bin/cat template.html`;
$secondfile = `/bin/cat data.txt`;

$firstfile = ~s/%%insert_here%%/$secondfile/g;
open(OUTFILE, ">foo.html") || die "Can't open $outfile for write: $!";
print OUTFILE $firstfile;
close OUTFILE;
[download]

--BigJoe

Learn patience, you must.
Young PerlMonk, craves Not these things.