Under strict, or more precisely, strict 'vars' , you have to declare each variable before you use it. You haven't declared $undefvar .

use strict;
use warnings;
my $newvar = 0;
my $undefvar;            # Declared, but undefined.
if (defined $undefvar) {
    $newvar = $undefvar;
}
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Update: refs → vars

($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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