It then occurred to me that the code won't run if the regexp has no capture groups!

It will run. The match returns 1 in such circumstances.

$ perl -Mv5.14 -e'if ( my @m = "b" =~ /(a)/ ) { say "@m"; }'

$ perl -Mv5.14 -e'if ( my @m = "a" =~ /(a)/ ) { say "@m"; }'
a

$ perl -Mv5.14 -e'if ( my @m = "b" =~ /a/   ) { say "@m"; }'

$ perl -Mv5.14 -e'if ( my @m = "a" =~ /a/   ) { say "@m"; }'
1
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