!/usr/bin/perl -w

# I most often use "-w" on first line the rather than "use warnings;".
# There can be difference later when using other modules.
# In this case, there is no difference.
#

if (! -d abc) 
{
    mkdir "abc", "0777" or die "Can't mkdir abc: $!";
}
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coud have been:

if (! -d "abc") 
{
    mkdir "abc", "0777" or die "Can't mkdir abc: $!";
}
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