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use strict;
use warnings;
...

my @idx = split /\s+/, $stdout;
print "$_\n" for @re[@idx];

my @re = (
  qw/./,
  qw/./,
  qw/.+/,
);

in $stderr:
tsort: -: input contains a loop:
...
tsort: -: input contains a loop:
tsort: 1
tsort: 2

hypothesis:
                  $re1 <= $re2
...
$str =~ /$re1/ and $str !~ /$re2/ -> falsifies hypothesis
$str !~ /$re1/ and $str =~ /$re2/ -> corroborates hypothesis
$str !~ /$re1/ and $str !~ /$re2/ -> no consequence