I know they are scary at first but this is the kind of thing regular expressions were born for.

The sample code below contains some comments which may help.

use warnings;

my @array = qw (A B C D);

## This example uses the open-file-handle-as-string feature
## Substitute \(my ...) for filepath
open (my $INFILE,"<", \(my $in = join "\n", qw(aA d DDD CDA e XYZ AABB
+CC)));
open (my $OUTFILE,">", \(my $out));

## Create a regular expression for all elements in the array.
## With the contents of array above the string created will be
## A+|B+|C+|D+
## Note how the '+' is suffixed on at the end because the 'join' inser
+ts 
## between array elements.
my $composite_elements_re = join ('+|',@array) . '+';


## Turn the string into a real regex with qr. 
## The 'x' modifier means you can add comments between the '/'s
## See perlretut etc in the perl documentation
my $complete_re = 
    qr/ $composite_elements_re /x
    ;

while (my $rec = <$INFILE>){
    ## The g modifier means get all matches.
    my @matches = $rec =~ /($complete_re)/gx ;
    ## If there are any matches print them to the output string (via f
+ilehandle!)
    ## Note that '@matches and' forces @matches to be interpreted as a
+ scalar (single) value.
    ## An array treated as a scalar returns the number of elements.
    ## So long as @matches >= 1 the matches will be put in the output.
    @matches and do { print $OUTFILE @matches };
}

close ($INFILE);
close ($OUTFILE);

## Change to print to file of your choice - or leave as stdout?
print "$out\n";
## prints ADDDCDAAABBCC
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