sub trim {
   $_ = $_[0];  # Assigns to $_, but $_ is aliased to " abc ".
   s/^\s+//g;
   s/\s+$//g;
   return $_;
}

# Fix:
# sub trim {
#    for ($_[0]) {  # foreach (unlike while) localizes the loop var.
#       s/^\s+//g;
#       s/\s+$//g;
#       return $_;
#    }
# }

print trim($_) for " abc ", " def ", " ghi "