I don't really understand开发者_高级运维 why the following piece of perl code
#!/usr/bin/perl -w
use strict;
use warnings;
strange($_) for qw(a b c);
sub strange {
open FILE, '<', 'some_file.txt' or die;
while (<FILE>) { } # this is line 10
close FILE;
}
Is throwing the following error
Modification of a read-only value attempted at ./bug.pl line 10.
Is this a bug? Or there is something I should know about the usage of the magic/implicit variable $_
?
The while (<fh>)
construct implicitly assigns to the global variable $_
.
This is described in perlop
:
If and only if the input symbol is the only thing inside the conditional of a while statement (...), the value is automatically assigned to the global variable $_, destroying whatever was there previously. (...) The $_ variable is not implicitly localized. You'll have to put a local $_; before the loop if you want that to happen.
The error is thrown because $_
is initially aliased to a constant value ("a"
).
You can avoid this by declaring a lexical variable:
while (my $line = <FILE>) {
# do something with $line
}
Yes, the while-loop reads into $_
which at that point is aliased to a constant (the string "a"). You should use local $_;
before the while-loop, or read into a separate variable.
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