As a preventative measure, this is a good rule of thumb to follow:
Whenever you aren't sure if a variable has been assigned a value, give it one explicitly =)

The || operator is very useful here.
Examples:

my $variable = $some_unknown_input || 'default';
my $foo = (defined $input ? $input : '');
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In the first example, if $some_unknown_input is not a true value, $variable will be assigned 'default'.
In the second example, if $input is undefined (note this is not necessarily the same as being non-true, as in the first example), it is assigned an empty string.