Evadently, there is problem copying file from a UNC path.

I don't see how it's evident, or even true.

C:\>md foo

C:\>md bar

C:\>copy nul foo\file
        1 file(s) copied.

C:\>perl -MFile::Copy -e"copy @ARGV" \\127.0.0.1\c$\foo\file \\127.0.0
+.1\c$\bar"

C:\>dir /b bar
file
[download]

How would it look in the actual script?

#!/usr/bin/perl

use strict;
use warnings;

use File::Copy qw( copy );

copy $ARGV[0], $ARGV[1];
[download]

It needs error checking.

If you need to hardcode a UNC path, you can either properly escape backslashes

my $path = '\\\\server\\share\\dir\\file';
[download]

or simply use forward slashes.

my $path = '//server/share/dir/file';
[download]

open has no problem with either.