$ 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