I think the code is "devolving". The code in original post seemed better to me.
maybe as JavaFan suggests calculate pi like:
my $PI = 2 * atan2(1, 0);
I would calculate $rad*$rad rather than $rad**2, pi*a0 is another possibility to factor out. There are some scaling things that you could do. But basically the code below is fine (2nd listing or this next one or combo).

Update: One obvious other thing is: Why push when you can print? Get rid of this push stuff and just output what you want, like this below. You use $r once, calculate some stuff and print it. In the loop below, there is a scaling factor of 1e-6, moving it out of this loop will do pretty much nothing. Below there is a simple integer "if" and a choice of 2 floating point equations. It won't get much better than this. 

foreach my $r (@radii) 
{
    my $rad = (1e-6)*$r;
    print "$rad ";
    if ($r>20)
    {
        my $sig = $twopi*($rad*$rad);
        print "$sig\n";
    }
    else
    {
        my $sig = $pi*($rad*$rad)*$a0*(1-exp((-$c0)*$rad));
        print "$sig\n";
    }
}

[download]
#3_3.pl
#loop over drop radius from 1 micron to 100 microns in steps of 2 micr
+ons
#use each radius to compute cross section
#if radius <20 microns, use (pi)*(r^2)*a*(1-exp(-c*r))
#if radius >= 20 microns, use 2*pi*(r^2)

# orginal post at:
# http://www.perlmonks.org/?node_id=771126

use strict;
use warnings;

my @radii = (1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31
+, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65,
+ 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 95, 97, 99);
my @rads = ();
my @sigs = ();

my $pi    = 3.14159;
my $twopi = (2)*(3.14159);
my $a0    = 1.18;
my $c0    = (0.28e6);

foreach my $r (@radii) 
{
    my $rad = (1e-6)*$r;
    push (@rads, $rad);
    if ($r>20)
    {
    my $sig = $twopi*($rad*$rad);
    push (@sigs, $sig);
    }
    else
    {
    my $sig = $pi*($rad*$rad)*$a0*(1-exp((-$c0)*$rad));
    push (@sigs, $sig);
     }
}
    
print "@rads \n";
print "@sigs \n";
__END__
Added some \n for formatting...
1e-006 3e-006 5e-006 7e-006 9e-006 1.1e-005 1.3e-005 1.5e-005 
1.7e-005 1.9e-005 2.1e-005 2.3e-005 2.5e-005 2.7e-005 2.9e-005 
3.1e-005 3.3e-005 3.5e-005 3.7e-005 3.9e-005 4.1e-005 4.3e-005 
4.5e-005 4.7e-005 4.9e-005 5.1e-005 5.3e-005 5.5e-005 5.7e-005 
5.9e-005 6.1e-005 6.3e-005 6.5e-005 6.7e-005 6.9e-005 7.1e-005 
7.3e-005 7.5e-005 7.7e-005 7.9e-005 8.1e-005 8.3e-005 8.5e-005 
8.7e-005 8.9e-005 9.1e-005 9.3e-005 9.5e-005 9.7e-005 9.9e-005 
9.05328279702527e-013 1.89602315397586e-011 
6.98230615994953e-011 1.56060261711467e-010 
2.76113310847353e-010 4.27940909754303e-010 
6.10048892522909e-010 8.21584452164295e-010 
1.06216829881331e-009 1.33170675846969e-009 
2.77088238e-009 3.32380222e-009 
3.9269875e-009 4.58043822e-009 
5.28415438e-009 6.03813598e-009 
6.84238302e-009 7.6968955e-009 
8.60167342e-009 9.55671678e-009 
1.056202558e-008 1.161759982e-008 
1.27234395e-008 1.387954462e-008 
1.508591518e-008 1.634255118e-008 
1.764945262e-008 1.90066195e-008 
2.041405182e-008 2.187174958e-008 
2.337971278e-008 2.493794142e-008 
2.65464355e-008 2.820519502e-008 
2.991421998e-008 3.167351038e-008 
3.348306622e-008 3.53428875e-008 
3.725297422e-008 3.921332638e-008 
4.122394398e-008 4.328482702e-008 
4.53959755e-008 4.755738942e-008 
4.976906878e-008 5.203101358e-008 
5.434322382e-008 5.67056995e-008 
5.911844062e-008 6.158144718e-008

[download]

In reply to Re: Improve my Code: Subroutines by Marshall
in thread Improve my Code: Subroutines by cheech

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.