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

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