use strict ;
use warnings ;

while (<DATA>) {
    my ($pat, $repl) = (split '/') [1, 2];
    # Escape $ and @ preceded by an even number of '\', except for $1 etc.
    $repl =~ s/(?<!\\)((?:\\\\)*(?:\$(?!\d)|\@))/\\$1/g ;
    my $e = "\$s =~ s/$pat/$repl/" ;

    my $s = "foo";
    print "$e  '$s' -> " ;

    eval $e ; die $@ if $@ ;
    print "'$s'\n" ;
}

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