Note that the error indicated

"number of files: 4" isn't numeric

rather than

"4" isn't numeric

Subtracting 2 is not the way to go. Some directories on some file systems don't have "." and "..". It's better to just filter them out.

# Filter out "." and ".."
my @array = grep !/^\.\.?\z/, readdir(DIR);
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# Filter out all "hidden" files
my @array = grep !/^\./, readdir(DIR);
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There's no reason to use a global variable for your directory handle. Change

opendir (DIR, "/some/path");
my @array = grep !/^\.\.\z/, readdir(DIR);
closedir (DIR);
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to

opendir (my $dh, "/some/path");
my @array = grep !/^\.\.\z/, readdir($dh);
closedir ($dh);
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Finally, you should have some error checking, if only something minimalistic for opendir

opendir (my $dh, "/some/path")
    or die("opendir: $!\n");
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