You can use the following notation to do that:

$_ .= ' an element' for @PRC_PRIV_ONLY;
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This iterates through the array, appending the string ' an element' to each array element.

This approach gives you a flat array. If you want to add an element to each line, you have to decide if it's satisfactory to just append the information to existing elements, or whether you really need a multi-dimensional data structure. For example:

@PRC_PRIV_ONLY = (
  'one string',
  'two strings',
  'three strings',
);
foreach my $element ( @PRC_PRIV_ONLY ) {
  $element .= ' appended data';
}
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Or if you need a 2d data structure:

foreach my $element ( @PRC_PRIV_ONLY ) {
  $element = [ $element, '2nd element' ];
}
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That transforms the above into:

@PRC_PRIV_ONLY = ( 
  [ 'one string',    '2nd element' ],
  [ 'two strings',   '2nd element' ],
  [ 'three strings', '2nd element' ],
);
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...which can be indexed like this: print $PRC_PRIV_ONLY[0][1], "\n"; (prints 2nd element).

Compared to the solution proposed by NetWallah, you are adding a temporary array and an additional processing step. This might have some impact on the memory footprint and performance of your program, but these things matter only if your input file is very large, not if you have a few hundreds or thousands lines.

Thanks for the helpful replies. I got it to work the way I need it to, and managed to address the warnings in regards to uninitialized variables and such. Here's the code for possible feedback:

open(FILE,"<",$DATA_FILE);

my @PRC_PRIV = <FILE>;

my @PRC_PRIV_ONLY = grep /Run as/ .. /^ /, @PRC_PRIV;


foreach $PRIV(@PRC_PRIV_ONLY) {

        chomp($PRIV);

        next if ($PRIV =~ m/Run|^ /);

       ($USER, $HOSTN, $CMDN) = split(" ", $PRIV, 3);

        if (defined($USER)) {

        print "$PRC_GRP,$USER,$HOSTN,$CMDN \n";

                }
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Any feedback welcome.

You said in the original post you wanted high performance. Try to replace this:

my @PRC_PRIV = <FILE>;

my @PRC_PRIV_ONLY = grep /Run as/ .. /^ /, @PRC_PRIV;
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with this:

my @PRC_PRIV_ONLY = grep /Run as/ .. /^ /, <FILE>;
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And measure if you get any performance improvement.

Another point is that you may want to include use strict; and use warnings; near the top of your program and declare your variables with the my operator. This is a recommended good practice which will save you a lot of debugging time.