$1 represent the captures of the last successful pattern match. In this instance, that would be s/one/ONE/. Since it has no captures, $1 and friends are empty. Copy $1 and friends when they still contain what you want, or avoid using $1 and friends completely.

my $line = "One two three four five";
if (my @captures = $line =~ m/(one)\s(two)\s(three)\s(four)\s(five)/i)
+ {
   $captures[0] =~ s/one/ONE/i;
   print "$_: $captures[$_-1]\n" for 1..5;
}
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Note that uc might be more appropriate here. Any maybe split.

my $line = "One two three four five";
if (my @captures = split(' ', $line)) {
   $captures[0] = uc($captures[0]);
   print "$_: $captures[$_-1]\n" for 1..5;
}
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