You can do that with a regular expression, but there's an easier way. You can just open your new file, write the <pre> tag, write your current file's contents, then write the </pre> tag.

my $input = 'existing.txt';
my $output = 'new.txt';
open ( my $in, '<', $input ) or die "Cannot open file $input for readi
+ng: $!\n";
open ( my $out, '>', $output ) or die "Cannot open file $output for wr
+iting: $!\n";
print $out "<pre>\n";
while ( <$in> ) {
    print $out $_;
}
print $out "\n</pre>\n";
close $out or warn "Error writing $output: $!\n";
close $in;
[download]

Compare the above to either keeping track of which line you're on from the input or slurping the input file into memory and doing a regex substitution over the whole thing. I think the KISS principle applies here.