open A, "<fileA" || die $!;
open B, "<fileA" || die $!;
open OUT, ">fileAtmp" || die $!;
my ($bufA, $bufB);
while ($bufA = <A>) {
  $bufB = <B> || last; #jump out if file B is shorter than A
  my @A = split ':', $bufA;
  my @B = split ':', $bufB;
  print OUT $bufA if $A[0] eq $B[0];
}
close A;
close B;
close OUT;