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#!/usr/bin/perl -l

...
my $range_sum    = ($range_start + $range_end) * $range_length / 2;

print $range_sum;

my $range_start = $ARGV[0];
$range_start++ if ( $opt_s );

if (    $ARGV[0] !~ m{ \A [+-]? \d+ \z }xms
     || $ARGV[1] !~ m{ \A [+-]? \d+ \z }xms ) {
    die "usage: $0 [-s] <integer> <integer>\n";
}