open(INFO, "<ptc.opt") or die "$!";

while ( my $line = <INFO> ) {
    next unless $line =~ /^GROUP/;
    chomp $line;
    $line =~ /GROUP ([^ ]*) (.*)/;
    push @{ $groups{$1} }, split( " ", $2 );
}

close INFO;

for my $key ( sort keys %groups ) {
    print "$key:\n";
    print join( " ", @{ $groups{$key} } ), "\n";
    print "\n";
}