You could try the following as it seems to cope with spaces between the b and it's newline and with b with an immediate newline by making the spaces optional and also making a character class that excludes newline, unlike \s.

Set up some text to try and set up the regular expression.

#!/usr/local/bin/perl
#
use strict;
use warnings;

our @tryThese = (
   "a 10\nb a2 s2\nc 30",
   "a 10\nb\nc 30",
   "a 10\nb  \nc 30",
   "a 10\nb a2 s2  \nc 30");

our $c;

our $rxAfterB = qr{(?m)^b[\x20\x09]*([^\n]*)};
[download]

Loop over @tryThese printing out what we are testing, doing the match then showing the result.

foreach my $a (@tryThese)
{
    print "\n\$a contains ...\n";
    {
        local $" = "<--\n";
        print "@{[split /\n/, $a]}\n";
    }
    ($c) = $a =~ /$rxAfterB/;
    print "c is -->$c<--\n\n";
}
[download]

When run this produces

$a contains ...
a 10<--
b a2 s2<--
c 30
c is -->a2 s2<--


$a contains ...
a 10<--
b<--
c 30
c is --><--


$a contains ...
a 10<--
b  <--
c 30
c is --><--


$a contains ...
a 10<--
b a2 s2  <--
c 30
c is -->a2 s2  <--
[download]

I hope this is of use.

Cheers,

JohnGG

Update: The slashes around the compiled regular expression in the line ($c) = $a =~ /$rxAfterB/; are superfluous. The line should read

    ($c) = $a =~ $rxAfterB;
[download]

JohnGG