This one uses the new(ish) treat-strings-as-file-handles feature (See Effective Perl Programming)

my $data = q(0       ASDF
        ASEE
        ASEE

13      DERG
        DREG

28      QWER
        QWER

42      WERT
        WERT
        WERT

55      QWEASD
        QWEASD
        QWEASD
        QWEASD
);

open $hndl , "<", \(my $s = $data);
open $out , ">", \(my $formatted = '');
my $substitute_field;

READ_FILE:
while (my $line = <$hndl>) { 
    chomp $line;
## Ignore any lines that don't contain relevant text
    $line =~ /\S+/ or next READ_FILE;

    $line =~ /^\s*(\d+)(\s+)([A-Z]+)/ and do {
        $substitute_field = $1;
        print $out $substitute_field, $2 ,$substitute_field,"\n";
        next READ_FILE;
    }; 
## If we don't have a field to substitute move on
    defined $substitute_field or next READ_FILE;
## Must have valid line
    $line =~ s/^(\s*)([A-Z]+)/$1$substitute_field/;
    print $out $line,"\n";
    
} 

print $formatted;
close $hndl;
close $out;
[download]

In:

0       ASDF
        ASEE
        ASEE

13      DERG
        DREG

28      QWER
        QWER

42      WERT
        WERT
        WERT

55      QWEASD
        QWEASD
        QWEASD
        QWEASD

out:

0       0
        0
        0
13      13
        13
28      28
        28
42      42
        42
        42
55      55
        55
        55
        55

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