The first element of the list returned by stat will be different on a directory and its parent directory if and only if the directory is a mountpoint.

#!/usr/bin/perl

use strict;
use warnings;

use Cwd qw/realpath/;
use File::Spec;

my $Dir = realpath(shift || File::Spec->curdir);
my $Parent = realpath(File::Spec->catdir($Dir, File::Spec->updir));

my $Dir_fsid = (stat($Dir))[0];
my $Parent_fsid = (stat($Parent))[0];

printf 'dir: [on %4x] %s%s', $Dir_fsid,$Dir,"\n";
printf ' up: [on %4x] %s%s', $Parent_fsid,$Parent,"\n";

if ($Dir_fsid == $Parent_fsid) {
  print "$Dir is not a mountpoint\n"
} else {
  print "$Dir is a mountpoint\n"
}

__END__
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