s/(...)/ func( $1 ) || $1 /ge;

s/(\[(...)\])/ func( $2 ) || $1 /ge;

s/(\[(...)\])/
    my $ret= func( $2 );
    defined $ret ? "[$ret]" : $1
/ge;

s/(\[(...)\])/
    my @ret= func( $2 );
    @ret ? "[@ret]" : $1;
/ge;