#!/usr/bin/perl
use strict;
use warnings;
use v5.20;

my $s = <<'ENDSTR';
aaa       : AAA
bbb       : BBB
ccc       : CCC
ENDSTR


my $m = 'bbb';

my $a = $1 if  $s =~ s/^$m *: (.*?)$/$1/rsm;
my $b = $1 if  $s =~ s/^$m *: (.*)$/$1/rm;

print "a: $a\n";
print "b: $b\n";