What I learned from this is to never assume that you know every detail of Perl syntax, so I won't be sure that I've covered all corner cases. Can you be?

I would not presume.

AFAIK the only characters that trigger interpolation are '$' and '@'. But, as you say, with Perl it's unsafe to assume that anything can be depended on.

However, if we can trust that much, then escaping '$' unless it's followed by a digit only lets past stuff which is supposed to be safe. And escaping '@' unconditionally seems to cover that. Unless there's some other magic that throws the escaping mechansm.

<sigh>

I suppose one could fall back on a hand affair:

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]) ? $tra
+p[$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 !"}/
[download]

which only attempts to simulate '$1'..'$9' -- but that may be sufficient. Of course, String::Interpolate may well do a more complete job.

Unless, of course, there's some other gotcha...