As others have said, the problem is probably that you don't have the full path name in your -f argument.

I like to use Path::Class because it avoids these problems:

use Path::Class qw(dir);
my $dir = dir("/path/to/dir");
my @files = $dir->children;

my $examplefile = $files[0] or die "no files";
print "is plain file" if -f $examplefile;

# Or:
print "is plain file" unless $examplefile->is_dir;
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