perl -e 'my $x = "abcdefg" =~ /(cd)/; print "$x\n";'
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prints

1

whereas matching in list context assigns the captures in the match

perl -e 'my ($x) = "abcdefg" =~ /(cd)/; print "$x\n";'
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prints

cd

Normally, or rather probably, you would be making more than one capture in the regular expression so you would do something like

my($this, $that) = $string =~ /abc(def).+?(pq)$/;
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Cheers,

JohnGG