You don't need a 3-argument open when the open doesn't have any user input in it. And obfuscating the code doens't really help either, even if the result is shorter.

use strict;
use warnings;

my ($argact, $handle, $num);

($argact) = $ARGV[0] =~ /(\d+)/;
exit if !$argact;

open $handle, 'one.txt';
$_ = join '', <$handle>;
close $handle;

($num) = m/(\d+)$/;

if ($num != $argact) {
    s/(\d+)$/$argact/;

    open $handle, '>one.txt';
    print $handle $_;
    close $handle;
}
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