my $string = 'string';
## s/string/replacement/;
$string->substitute( 'pattern', 'replacement' );
## s/string/codehere/e;
$string->substitute( 'pattern', sub { ... } );
## s/string/codehere/ee;
$string->substitute( 'pattern', sub { eval ... } );
## s/string/codehere/eee;
$string->substitute( 'pattern', sub { eval eval ... } );