You could add --test or -t when running on the command line like this

#!/usr/bin/perl -w
use strict;
use CGI qw(-oldstyle_urls :standard);
use CGI::Carp qw(fatalsToBrowser);
use Date::Calc qw( Add_Delta_Days Decode_Date_US Today );

# no html option
use Getopt::Long;
my $test = 0;
GetOptions('test'=> \$test);

# yesterday's date
my @today = ($ARGV[0]) ? Decode_Date_US( $ARGV[0] ) :  Today() ;
my @yesterday = Add_Delta_Days( @today, -1 );
my $t_string = sprintf "%04d-%02d-%02d", @today;
my $y_string = sprintf "%04d-%02d-%02d", @yesterday;

# create page
my $q = new CGI;
my $page = "\n\n";

# switch ouput format if test option
if ($test){
  my $line = '-' x 80;
  $page .= sprintf " %-16s %-22s\n", "TODAY", "YESTERDAY";
  $page .= ' '.$line."\n ----------$t_string  - $y_string-----------\n
+";
} else {
  $page = qq!<table width="100%" border="0" bgcolor="#ffffff" cellpadd
+ing="0" cellspacing="0">
  <tr>
    <td width="60%">Date Format Sample:</td>
    <td width="40%" align="left"><b>$y_string</b></td>
  </tr>
  <tr>
    <td width="100%" colspan="2">&nbsp;</td>
  </tr>
</table>!;
}
print $page;
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