#! perl -slw
use strict;

sub morefoo{ $_[0] * 3 }

sub a {
    my $x = shift;
    return defined $x ? morefoo( $x ) : undef;
}

sub b {
    my $x;
    return defined( $x = shift ) ? morefoo( $x ) : undef;
}
sub c {
    return morefoo( shift // return undef );
}

print "$_ :> ", a( $_ ) for undef, 0, 1 ;
print "$_ :> ", b( $_ ) for undef, 0, 1 ;
print "$_ :> ", c( $_ ) for undef, 0, 1 ;
__END__
C:\test>junk93
Use of uninitialized value $_ in concatenation (.) or string at C:\test\junk93.pl line 21.
Use of uninitialized value in print at C:\test\junk93.pl line 21.
 :>
0 :> 0
1 :> 3
Use of uninitialized value $_ in concatenation (.) or string at C:\test\junk93.pl line 23.
Use of uninitialized value in print at C:\test\junk93.pl line 23.
 :>
0 :> 0
1 :> 3
Use of uninitialized value $_ in concatenation (.) or string at C:\test\junk93.pl line 25.
Use of uninitialized value in print at C:\test\junk93.pl line 25.
 :>
0 :> 0
1 :> 3