There's not a built-in. The regexp approach is fine. If you want it to be a sub, just put it in a sub definition:

sub trim_white {
   my $string = shift;
   $string =~ s/^\s+//g;
   $string =~ s/\s+$//g;
   return $string;
}

# Usage example:
my $trimmed = trim_white( $untrimmed );
[download]

Update: For completeness's sake, it might be worth noting that on CPAN you can find String::Util, which includes a function called trim() that does exactly what you're talking about. ...it's been done. ;)