According to the counting rules (rule 1 above) you only count the characters inside the subroutine block (for one liners you count the whole command line). So yours is 18 chars.

Not to spoil anyone's fun, but here's a small tip for shaving characters --- use pop rather than $_[0], that gives us a 16 character solution:

sub S {(pop=~/\w+/g)[4]}
       123456789_123456   # 16
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