First - working code:

use strict;
use warnings;

my @config = qw|this oldtext needs replacement but not newtext Just ol
+dtext|;

for (@config) {
   s/oldtext/newtext/
}

print qq|$_;\n| for @config; # "Perl" style print

for my $t (0 .. $#config){  # Your style (c-style for loop)
  print "$t:$config[$t];\n";
}
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Now - the stuff that appears not to have worked for you:

@config = map {s/oldtext/newtext/g; $_; } @config;
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This works, but is way overkill.

First, omit the "@config =".

Then omit the $_. This works as well:

  map {s/oldtext/newtext/g; } @config;
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but then you may as well use the simpler:

 s/oldtext/newtext/ for @config;
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Which avoids generating a throw-away array, and is almost the same as your second attempt. (I would not use the "g" modified unless you were expecting multiple instances of the search string in one element of the array.)

Your print loop is attempting to apply two indexes to a single-dimension array.