This is not a bug in File::Copy but more likely an error in your understanding of how your shell launches programs and parses command line parameters. You can confirm this by printing out the parameters your program gets:

#!/usr/bin/perl -w
use strict;
print "Using '$ARGV[0]' as source\n";
print "Using '$ARGV[1]' as target\n";
print "Leftover arguments: '$ARGV[2]'\n";
[download]

Depending on what shell you're using, reading the manual on "quoting" helps, but alternatively, most shells use quoting rules not entirely unlikely to Perl's own rules. In short, if you want to include a blank in a command line argument, you need to quote it in the shell.