join lines after colon (perl)

Question

I have this lines:

alpha: beta
beta: alpha, beta
omega: beta, gamma, alpha
gamma: 
alpha
beta
gamma
epsilon: alpha

I want to join line contains

Answer 1

This should do the same as RCs code, but a little bit shorter:

my @lines;
while() {
    chomp;
    if(m/^\w+:\s(\w+(,\s)?)*$/) {
            push @lines, $_;
    } else {
            $lines[$#lines] .= ", " unless($lines[$#lines] =~ m/^\w+:\s?$/);
            $lines[$#lines] .= $_;
    }
}
print join "\n", @lines;