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;