Here's a basic idea of how I might approach this problem.

1. Open 1.txt and read its contents into a data structure suitable for later use.
2. Process each line of 2.txt. Change the line based on the data read in step 1, and write it to a third file.
3. When processing of 2.txt is complete, rename the third file to 2.txt

Example of step 1:

my %new_pass;
while (<F1>) {
  chomp;
  my @f = split(':', $_);
  $new_pass{$f[0]} = $f[2];
}
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Example of step 2:

while (<F2>) {
  chomp;
  my @f = split(':', $_);
  if (exists $new_pass[$f[0]]) {
    $f[1] = $new_pass[$f[0]];
  }
  print F3 join(':', @f), "\n";
}
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