How come undef eq '' ??

tmharish has asked for the wisdom of the Perl Monks concerning the following question:

I, accidentally stumbled on the fact that undef eq ''

( Leaving out strict and warnings is intentional )

perl -e 'if( undef eq "" ) { print "In\n" } else { print "Out\n" }'
In

perl -e 'my $tmp; if( $tmp eq "" ) { print "In\n" } else { print "Out\
+n" }'
In


perl -e 'my $tmp; if( $tmp ne "" ) { print "In\n" } else { print "Out\
+n" }'
Out

perl -e 'if( undef ne "" ) { print "In\n" } else { print "Out\n" }'
Out
[download]

Is there some reasoning for this?

The problem with this is that when I want to clean up warnings I add $tmp and:

perl -e 'my $tmp; if( $tmp and ( $tmp eq "" ) ) { print "In\n" } else 
+{ print "Out\n" }'
Out
[download]

The outcome is to throw the entire program off!

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