Here is another way using Tie::File

use Tie::File;
use strict;
my (@A, @B, @C); my ($field1, $A, $B);

tie @B, 'Tie::File', 'txtA.txt' or die "couldn't TiE $!\n";
tie @A, 'Tie::File', 'txtB.txt' or die "couldn't TiE $!\n";

foreach $A (@A) {
    my ($field1) = split/:/, $A;
    print $field1;

foreach $B (@B){
    if($B=~/^$field1/){ 
    #found match push it onto C
    push @C, $A;}
  }
}
## We remove those that didn't match by replacing @A with @C :)
@A = @C;
print @A;
[download]

Here are the test files
txtA.txt

filea:file2:file1
file2:something:old
[download]

txtB.txt

fileB:file2:file3
filea:file2:file1
file2:filex:file23
file34:file2:file3
filea:file2:file1
file2:filex:file23
[download]

Cheers, James Edit:removed a label