How to deal with $$genecolor issue
You're trying to assemble scalar variable names at runtime. Don't do that. Rather, change your sub to
my %color = ( lavender => $im->colorAllocate(230,230,255),
lightblue => $im->colorAllocate(173,216,230),
honeydew => $im->colorAllocate(240,255,240) );
while (@ary) {
my $genecolor= coloring ();
$im->filledRectangle($X1,$Y1-20,$X2,$Y2,$color{$genecolor});
Other things which could be improved in your code: use for instead of while when iterating over an array, use scalar @color instead of $#color in your subroutine, pass the array of possible colors into the subroutine instead of maintaining it in two places in your code. E.g.
sub coloring{
my @color= @{$_[0]};
my $color_no=int(rand(scalar @color));
return $color[$color_no];
}
my %color = ( lavender => $im->colorAllocate(230,230,255),
lightblue => $im->colorAllocate(173,216,230),
honeydew => $im->colorAllocate(240,255,240) );
for (@ary) {
my $genecolor= coloring ([keys %color]);
$im->filledRectangle($X1,$Y1-20,$X2,$Y2,$color{$genecolor});
}