my $string = "123A1234";
my $regex  = qr/((?<=\d+))(A)\1(.*)/;

my ($num, $letter, $leftover) = $string =~ $regex;
ok( $num eq '123' );
ok( $letter eq 'A' );
ok( $leftover eq '4' );

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my $rev_string = "4321A321";
my $rev_regex  = qr/.../; # <-- what goes here??

my ($leftover, $letter, $num) = $rev_string =~ $rev_regex;

ok( $num eq '123' );
ok( $letter eq 'A' );
ok( $leftover eq '4' );