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mkdir "$dir", 0777 || die "Could not mkdir \"$dir\": $!\n";

my $dir= "source"; # A directory that I know exists.
warn "Using or...\n";
...
warn "Using ||...\n";
mkdir "$dir", 0777 || die "Could not mkdir \"$dir\": $!\n";
warn "Done.\n";

E:\etm\Work>perl -w mkdir.pl
Using or...
Could not mkdir "source": File exists
Using ||...
Done.