# Relative to the current dir on the current drive
open(my $fh, '<', 'file')
open(my $fh, '<', './file')
open(my $fh, '<', '../file')
open(my $fh, '<', '../dir/file')
open(my $fh, '<', 'dir/file')

# Relative to the current dir on the drive c:
open(my $fh, '<', 'c:file')
open(my $fh, '<', 'c:./file')
open(my $fh, '<', 'c:../file')
open(my $fh, '<', 'c:../dir/file')
open(my $fh, '<', 'c:dir/file')