in reply to Assigning default values to function arguments which may be “empty”

Checking the length is a common way. If you are concerned about a new variable, use the one Perl already created for you: 
sub f {
    length $_[0] ? $_[0] : 'default_value';
}

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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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In Section Seekers of Perl Wisdom