$id = $Words_db{$word};
	
foreach $i (keys %Index_db) {
	if ($i == $id) {
		@fileId = split( /:/, $Index_db{$i});
		foreach $fId (@fileId) {
			# ...
		}			
	}
}	

##</code><code>##

my $piece;

foreach (keys %Words_db) { 
	if ( ... ) {
		# if $piece is a substring of $_
		...
	} else {
		$piece does not occur in $_
		...
	}
}