Update: stepped through the execution and found why it fails.
In require_ok

 # Try to determine if we've been given a module name or file.
    # Module names must be barewords, files not.
    $module = qq['$module'] unless _is_module_name($module);
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the problem is in _is_module_name
foo is being treated as a module because of this

    $module =~ s/\b::\b//g;

return $module =~ /^[a-zA-Z]\w*$/ ? 1 : 0;
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with "foo.pl" it returns 0
with "foo" it returns 1
changing it to

#    $module =~ s/\b::\b//g;

return $module =~ /^[a-zA-Z]\w*$/ ? 1 : 0 if $module =~ s/\b::\b//g;
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resolves the problem and works for these tested scenarios

require_ok("foo");
require_ok('foo');
require_ok("foo.pl");
require_ok('foo.pl');
require_ok("Net::FTP");
require_ok('Net::FTP');
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