Array got flushed after while loop within a filehandle

Question

I got a problem with a Perl script.

Here is the code:

use strict;
use Data::Dumper;

my @data = ('a','b');

if(@data){
  for(@data){
    &debug_check($_)
  }
}

print "@data";#Nothing is printed, the array gets empty after the while loop.

sub debug_check{
  my $ip = shift;
  open my $fh, "<", "debug.txt";
  while(<$fh>){
    print "$_ $ip\n";
  }
}

Array data, in this example, has two elements. I need to check if the array has elements. If it has, then for each element I call a subroutine, in this case called "debug_check". Inside the subroutine I need to open a file to read some data. After I read the file using a while loop, the data array gets empty.

Why the array is being flushed and how do I avoid this strange behavior?

Thanks.

Answer 1

The problem here I think, will be down to $_. This is a bit of a special case, in that it's an alias to a value. If you modify $_ within a loop, it'll update the array. So when you hand it into the subroutine, and then shift it, it also updates @data.

Try:

my ( $ip ) = @_; 

Or instead:

for my $ip ( @array ) { 
     debug_check($ip);
}

Note - you should also avoid using an & prefix to a sub. It has a special meaning. Usually it'll work, but it's generally redundant at best, and might cause some strange glitches.

Answer 2

while (<$fh>)

is short for

while (defined($_ = <$fh>))

$_ is currently aliased to the element of @data, so your sub is replacing each element of @data with undef. Fix:

while (local $_ = <$fh>)

which is short for

while (defined(local $_ = <$fh>))

or

while (my $line = <$fh>)   # And use $line instead of $_ afterwards

which is short for

while (defined(my $line = <$fh>))

Be careful when using global variables. You want to localize them if you modify them.