A little more wordy than the other solution, but in this case perhaps easier to understand.

use strict;
use warnings;

my @matches;
while (<DATA>) {
    # MAN followed by at least one number
    while (m/MAN((\D\d+)+)/ig) {
        # Trim leading non-digit, split rest of number string into ind
+ividual numbers
        my $n = $1;
        $n =~ s/^\D//;
        push @matches, split /\D/, $n;
    }
}
print join ',', @matches;

__DATA__
MAN:113 some other test 666....
MAN 231,456 TEXT
text alpha Man 999 etc.. 77789 etc
man 111,222,333
[download]

Output: 113,231,456,999,111,222,333