Modification of a read-only value attempted

ropey has asked for the wisdom of the Perl Monks concerning the following question:

Could someone explain to me what causes this behaviour ?

#!/usr/bin/perl -w

use strict;

my @subs = ("_iata_city");


foreach my $sub(@subs) {
    $sub =~ s/^_//g;
    print $sub;;
}
foreach my $sub(qw/_iata_city/) {
    $sub =~ s/^_//g;
    print $sub;;    
}
[download]

1) First loop = iata_city
2) Second loop = Modification of a read-only value attempted
I am assuming it has something to do with the _ in the qw// but am not sure why ?

Comment on Modification of a read-only value attempted Download Code

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Re: Modification of a read-only value attempted by kyle (Abbot) on Dec 03, 2008 at 14:23 UTC
In each case, the variable, `$sub`, is aliased to what's in the parens on the `foreach` line. In the second case, that's a read-only value, not a variable. For more, have a look at Common Causes for "Modification of a read-only value attempted".	[reply] [d/l] [select]
Re: Modification of a read-only value attempted by almut (Canon) on Dec 03, 2008 at 14:25 UTC
In addition to what the A.M. said, it's maybe worth pointing out that `$sub` is not a copy of, but rather an alias to the elements being iterated over — which explains why the modification is being attempted on the constant `"_iata_city"` itself.	[reply] [d/l] [select]
Re: Modification of a read-only value attempted by Anonymous Monk on Dec 03, 2008 at 14:19 UTC
In the first case, $subs[0] is a variable you can freely edit. In the second case, '_iata_city' is a constant, which you can't modify. Nothing to do with underscores, just constants vs variables as the error message says.	[reply]