At least on my 5.28, the difference is negligible:

use warnings;
use strict;
use Benchmark 'cmpthese';

cmpthese(-2, {
    predecl => sub {
        my $y;
        my $x;
        for $x (1,2,3) { $y+=$x }
    },
    lexical => sub {
        my $y;
        for my $x (1,2,3) { $y+=$x }
    },
});

__END__

             Rate predecl lexical
predecl 9175035/s      --     -1%
lexical 9275893/s      1%      --

[download]
But wakes in me primordial fears

Yes, I know the feeling well. But my philosophy has become: first, code so that it works, avoiding only the really obvious performance mistakes (like scanning an array instead of using a hash and the like). Then, if it's fast enough for your puproses, you're done. But if you want to optimize, remember that optimization is a science: measure the performance, identify the hotspots, benchmark the alternatives, modify the code accordingly, measure the difference in performance, and repeat until the performance becomes good enough for your purposes.

In reply to Re: declaring lexical variables in shortest scope: performance? by haukex
in thread declaring lexical variables in shortest scope: performance? by bliako

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