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;
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The rest is left as an exercise for the reader. You should get the gist.