perl - "Modification of a read-only value" on while loop with implicit $_ variable -

i don't understand why following piece of perl code

#!/usr/bin/perl -w  use strict; use warnings;  strange($_) qw(a b c);  sub strange {   open file, '<', 'some_file.txt' or die;   while (<file>) { } # line 10   close file; } 

is throwing following error

modification of read-only value attempted @ ./bug.pl line 10. 

is bug? or there should know usage of magic/implicit variable $_?

the while (<fh>) construct implicitly assigns global variable $_.

this described in perlop:

if , if input symbol thing inside conditional of while statement (...), value automatically assigned global variable $_, destroying whatever there previously. (...) $_ variable not implicitly localized. you'll have put local $_; before loop if want happen.

the error thrown because $_ aliased constant value ("a").

you can avoid declaring lexical variable:

while (my $line = <file>) {     # $line }