my $LINE_PATTERN = qr{
    ([^:]+)  # capture everything before ...
    :\s*     # the colon and any newline or other whitespace,
    ([^\n]+) # then capture everything before
    \n       # the next newline
}msx;  

my $app_text = "one:partridge\ntwo:\nturtle doves\nthree:french hens\n";

while ($app_text =~ /$LINE_PATTERN/g) {
    print "$1: $2\n";
}