Why does perl warn that open my $fh, $file is missing parentheses?

It is my first day to perl, and I find this warning very confusing.

Parentheses missing around "my" list at ./grep.pl line 10.

It seems

open FILE, $file;

works fine.

What is wrong with

open my $fh, $file;

Thanks!

#!/usr/bin/perl

use strict;
use warnings;

sub grep_all {
        my $pattern = shift;

        while (my $file = shift) {
                open my $fh, $file;
                while (my $line = <$fh>) {
                        if ($line =~ m/$pattern/) {
                                print $line;
                        }   
                }   
        }   
}

grep_all @ARGV;