use strict ;
use warnings ;

while (<DATA>) {
    my ($pat, $repl) = (split '/') [1, 2];

    my $s = "foo" ;
    printf "%40s '%s' -> ", "/$pat/$repl/", $s ;

    my @trap = $s =~ m/$pat/ ;
    $repl =~ s/(?<!\\)((?:\\\\)*)\$([1-9])/(defined($1) ? $1 : '')
                                         .(defined($trap[$2-1]) ? $trap[$2-1] : '')/eg ;
    $s =~ s/$pat/$repl/ ;

    print "'$s'\n" ;
}

__DATA__
s/foo/bar/
s/(fo)o/bar$1/
s/(fo)o/bar\$1/
s/(fo)o/bar\\$1/
s/oo/${system "echo hello sailor !"}/
s/oo/@{system "echo hello sailor !"}/