http://www.perlmonks.org?node_id=633574

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Most painful lesson learnt:

Allocation and assignment don't always happen together in a 'my' statement. Subroutine scratchpads can persist and hold on to old values.

my $var = 'value' if $condition; # this can bite

This is better written as:

my $var = $condition ? 'value' : undef; # assignment always happens