While it's not as powerful as using the debugger itself, I use a small block based around eval to achieve mostly the same thing:

    while (1)
    {
        print "$0> ";
        chomp(my $input = scalar <STDIN>);
        last if $input =~ m/^(q|quit)$/;
        my $res = eval $input;
        chomp(my $out = $@ ? $@ : $res);
        print "$out\n";
    }
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