There are probably a bunch of regexes you can use to do this with the /s flag, but I would just do it programmatically:

#!/usr/bin/perl -w
my $output = '';
while(<DATA>){
    if(/\:$/){$output .= qq|\n$_|} else {chomp($_);$output .= $_ . q|,
+|};
}
print $output;
1;
__DATA__
heading1:
this
is
a
tst
heading2:
this
is
another
test

The result is:

heading1:
this,is,a,tst,
heading2:
this,is,another,test,
C:\Temp>
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