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$ perl -Mwarnings -le 'my $x; print 1 if $x == undef'
Use of uninitialized value in numeric eq (==) at -e line 1.
Use of uninitialized value $x in numeric eq (==) at -e line 1.
1

$ perl -Mwarnings -le 'my $x; print 1 if defined $x'

$ perl -Mwarnings -le 'my $x; print 1 if ! defined $x'
1