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 "

##</code><code>##

sub f {
   my $pos = tell(DATA);
   while (<DATA>) {  # Assigns to $_, but $_ is aliased to 1.
      print;
   }
   seek(DATA, $pos, 0);
}

# sub f {
#    my $pos = tell(DATA);
#    while (defined(my $line = <DATA>)) {
#       print $line;
#    }
#    seek(DATA, $pos, 0);
# }

f() for 1,2,3;

__DATA__
a
b
c