foreach my $chars (split(//,$input_string)) {
    $hexchars .= sprintf "%x", ord($chars);
}
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Be aware that the format %x will cause you problems if your string contains characters with ordinals in the range 0x00 through 0x0f. Consider the following code where the string is terminated with CRLF.

$ perl -le '
$str = qq{test\r\n};
$wrong = join q{}, map { sprintf q{%x}, ord } split m{}, $str;
print $wrong;
$right = join q{}, map { sprintf q{%02x}, ord } split m{}, $str;
print $right;'
74657374da
746573740d0a
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You can see that %x will use a variable width depending on the value to be formatted and so, for single digit values, doesn't show a leading zero. Conversely, %02x does because it is specifying a width of 2 characters, padding with leading zeros as necessary.

I hope this is of interest.