Something like this?

use strict;
use warnings;

my @data = split /\n/, <<'DATA';
324K    324K
440K    533K
23T    224G
42G    42G
1.9T    709G
294K    294K
684K    684K
492K    492K
62M    64M
48K    41M
34M    433K
317K    812K
DATA

my %units = (
    K  => 1024**1,
    M  => 1024**2,
    G  => 1024**3,
    T  => 1024**4,
);

my $unit_re = join '|', sort keys %units;
$unit_re = qr/$unit_re/;

my $number_re = qr/[0-9]+(?:\.?[0-9]+)/;

foreach (@data) {
    warn "Line seems malformed: $_" && next
        unless /^\s*($number_re)($unit_re)\s+($number_re)($unit_re)\s*
+$/;
    
    my $diff = $3 * $units{$4} - $1 * $units{$2};
    
    print "Diff: $diff\n";
}
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