my %myHash = (hello=>"Howdy", goodbye=>"See ya");

while (<DATA>)
{
	s/\$(\w+)/$myHash{$1}/eg;
	print;
}
__DATA__
When I want to say hello I say $hello.
When I want to say goodbye I say $goodbye!

##</code><code>##

s/\$(\w+)/dbLookup($1)/eg;

sub dbLookup {
# Do database lookup (which would take a few more lines!)
return $lookedup ? $lookedup : '';
}