Are you sure you're using the range operator correctly? Your code will return the elements of @old_array from the occurrence of $first_val until the occurrence of $second_val, inclusive. Whereas a compare like jaa's will return the elements whose values are between $first_val and $second_val.

use strict;
my $first_val = -3;
my $second_val =  -0.0999999999;
my @old_array = <DATA>;
print "Range operator\n";
my @new_array = grep {$_ == $first_val .. $_ == $second_val} @old_arra
+y;
print @new_array;
print "\nComparison\n";
@new_array = grep {$_ >= $first_val and $_ <= $second_val} @old_array;
print @new_array;
__DATA__
-5
-3
7
0.0999999999
-0.0999999999
-2
-6
__OUTPUT__
Range operator
-3
7
0.0999999999
-0.0999999999

Comparison
-3
-0.0999999999
-2
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flounder