I'm sure you've gotten all the info you needed about eval, but if you use lookaheads, you could make this work without needing to specify captures (with parentheses) in your $pattern. (I'm assuming you don't want to define $repl based on user input, but it could be made to work that way too)

#untested, but should work
my $str = 'abcd';
my $pattern = shift;

if ($str =~ $pattern) {
    my $repl = shift;

    say '$repl: ', $repl;
    say 'old $str: ', $str;

    $str =~ s/(?=$pattern)/$repl/;
    say 'new $str: ', $str;
}
__END__
$ perl 2perl.pl 'ab' '--'
$repl: --
old $str: abcd
new $str: --abcd
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@_=qw;
Just another Perl hacker,;
;$_=q=print
"@_"= and eval;
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