#!/usr/bin/perl -w
use strict;

my($amt, $units, $availableunits) = @ARGV[0,1], 'kmgt';
$units =~ s/^([$availableunits])(b)?$/
        (1000*($2?1.024:1))**index("X$availableunits",$1)
/exi;

print $amt * $units;
print "\n";
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Give two arguments on the command line, the first being a number, the second being k, m, g or t, with an optional trailing b.