This line is checking if the key $stuff[0] exists in the hash %nums.

$stuff[0] is the first element in the array @stuff.

Example:

my @stuff = qw/a b c d/;
my %nums = ( a => 1,
             b => 2,
             c => 3,
           );

if (exists $nums{$stuff[0]}) {
  print "$stuff[0] is in \%nums"; )
}
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This will print a is in %nums.