# <SNIP> - handle some pre-processing

# $FILE contains a safe path to an existing file on
# the filesystem, based on browser input.
open( my $fh, '<', $FILE ) or die "open failed: $!";
my $file = do { local $/; <$fh> };
close( $fh );

my ($needed) = $file =~ /\A<!\-\-(.*?)\-\->/s;

# now output the document, using the $needed info.