replace '<$file' with "<$file" fix my problem. Thanks a lot for your suggestion.

--here is my original question--

Hello, I am trying to copy multiple files to their corresponding output file as following:

use warnings;
use strict;
use IO::File;
use diagnostics;

my @handles;
my @temp;

foreach my $file ( glob( '*.input' ) ) {
        (my $out = $file) =~ s/\.input$/.output/;
        push @handles, [
                (IO::File->new('<$file') || die "die here\n"),
                (IO::File->new('>$out') || die "die under\n"),
        ];
}
#push @handles, [ (IO::File->new('</etc/hosts') || die "die here\n"), 
+(IO::File->new('>/tmp/hostlist') || die "die under\n"), ];

while (@handles) {
  @handles = grep {
    if (defined(my $line = $_->[0]->getline)) {
      print { $_->[1] } $line;
    } else {
      0;
    }
  } @handles;
}
[download]

It complains "Uncaught exception from user code:". If I don't use foreach loop, only copy one file as indicated in the commented out line. It is working fine.

What is wrong with the loop? Thanks in advance!