maybe you could consider putting the radii and the cross sections in the same array as they seem to belong together.

#!/usr/bin/perl -w

# loop over drop radius from 1 micron to 100 microns in steps of 2 microns
# use each radius ($r) to compute cross section
# if r < 20 microns,  use (pi)*(r^2)*a*(1-exp(-c*r))
# if r >= 20 microns, use 2*pi*(r^2)
use strict;
use Data::Dumper;

my @cross_sections = ();
my ($pi, $a, $c) = (atan2(0, -1), 1.18, 0.28e6);

for (my $i = 1 ; $i < 100; $i += 2) {
    my $r = $i / 1000000;
    if ($i < 20) {
        push @cross_sections, [$r, $pi * $r**2 * $a * (1 - exp(-$c * $r))];
    } else {
        push @cross_sections,[$r, 2 * $pi * $r**2];
    }
}

print Dumper(\@cross_sections);