$str =~ s{$pat}{
   prompt( $_, $-[0], $+[0]-$-[0], $&, $repl, $`, $' )
   ? $repl
   : $&
}eg

##</code><code>##

use String::Substitution qw( interpolate_match_vars last_match_vars );

$str =~ s{$pat}{
   my $true_repl = interpolate_match_vars( $repl, last_match_vars() );
   prompt( $_, $-[0], $+[0]-$-[0], $&, $true_repl, $`, $' )
   ? $true_repl
   : $&
}eg