You doesn't use File::stat module properly. Calling stat($filename) returns reference to the stat object. So this is the way you should use it:

#!/usr/bin/env perl
use strict;
use warnings;

use File::stat;
my $filename = $ARGV[0];
my $stat_obj = stat($filename)
    or die "Can't stat $filename";

print "$filename at " . $stat_obj->mtime() . "\n";
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