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});
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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];
}
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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});
}
[download]