Before anything else, let's change the variable names. $a and $b are reserved for sort, lexicalizing them could lead to strange behaviour.

There are several ways how to prevent the warnings. The most common one is to test whether there was a match: 

if (my ($x, $y) = 'ab' =~ /(x)(y)/) {
    print "$x$y\n";
}

[download]

Another possibility it to use the convenient defined-or operator (Perl 5.10+ needed): 

my ($x, $y) = 'ab' =~ /(x)(y)/;
print $x // q(), $y // q(), "\n";

[download]
($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,

[download]

In reply to Re: Idiom for setting variables to empty string if capture is undefined? by choroba
in thread Idiom for setting variables to empty string if capture is undefined? by nysus

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