I am not an expert on this, but I do think I have an idea of what's going on: AFAIK the state that m//gc keeps is attached to each string (pos). See the output of the following, just your test cases with debugging added:

use warnings;
use strict;
use Carp;
use Devel::Peek;

sub parse {
    Dump $_[0];
    ${$_[0]}=~/\Gfoo/gc or confess;
}

warn "##### Case 1 #####\n";
parse \(''.'foo') for 1..2;

warn "##### Case 2 #####\n";
parse \('foo') for 1..2;

[download]

In "Case 1", the dot operator (concat) appears* to create a new string on each execution of the loop. In "Case 2", the same string (same memory address etc.) is passed to the function each time, so m/\G.../gc keeps its state, and so the second call fails.

As for B::Deparse, from its docs:

The output of B::Deparse won't be exactly the same as the original source, since perl doesn't keep track of comments or whitespace, and there isn't a one-to-one correspondence between perl's syntactical constructions and their compiled form, but it will often be close.

* Update: Added the "appears to", since my further investigation below has made me uncertain as to what is going on the exact technical explanation is.

In reply to Re: What's going on with either constants folding or B::Deparse output in this case? (updated) by haukex
in thread What's going on with either constants folding or B::Deparse output in this case? by vr

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