use strict;
use warnings;
my %data = (
  first_name => 'Foo',
  email => 'Bar',
);
$/ = undef;
my $s = <DATA>;
$s =~ s/\$([A-Z_]+)/exists $data{lc $1}?$data{lc $1}:$&/esg;
print $s;

__DATA__
  First name is $FIRST_NAME and<br>
  email is: $EMAIL.
  junk is $JUNK