$ perl -Mwarnings -le " my ( $l, $r ) = split /\s/, 'a b',2; print qq[$l$r\n]; "
ab


$ perl -Mwarnings -le " my ( $l, $r ) = split /\s/, 'a',2; print qq[$l$r\n]; "
Use of uninitialized value $r in concatenation (.) or string at -e line 1.
a


$ perl -Mdiagnostics -Mwarnings -le " my ( $l, $r ) = split /\s/, 'a',2; print qq[$l$r\n]; "
Use of uninitialized value $r in concatenation (.) or string at -e line 1 (#1)
    (W uninitialized) An undefined value was used as if it were already
    defined.  It was interpreted as a "" or a 0, but maybe it was a mistake.
    To suppress this warning assign a defined value to your variables.

...*snip*...

a


##</code><code>##

no warnings 'uninitialized';