That's exactly it (although there could be 0 elements if the match fails).

my @nums = ($x =~ /\s(\S+)/)[0] =~ /(\d+)/g;
could also be written as
my @nums = ($x =~ /\s(\S+)/ ? $1 : undef) =~ /(\d+)/g;

If you're going to use /g, drop the \s:

# @nums = ('3', '33');
my $word = 2;
my @nums = ($x =~ /(\S+)/g)[$word] =~ /(\d+)/g;
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or use split:

# @nums = ('3', '33');
my $word = 2;
my @nums = (split(' ', $x))[$word] =~ /(\d+)/g;
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