You talk about characters - when using UTF-8, length and substr count characters, not octets, so in the output, you can find more than 21 octets. If you were already talking about characters, not octets, can you please show some short example input that exhibits the problem, preferrably together with a hexdump of the relevant portion of the file?

If you want to extract a given number of bytes from a UTF-8 string, use bytes::substr :

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use open IO => ':encoding(UTF-8)', ':std';

my $string = join q(), map chr, 9312 .. 9321;
say $string;
say substr $string, 0, 7;
say bytes::substr $string, 0, 7;
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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it doesn't count the correct # of characters.

With no sample data, that's pretty hard to verify. Have you read length() miscounting UTF8 characters? There's lots of good info in there (length works analagously to substr) along with easy examples of how to provide code complete with data which demonstrates the issue.

Year*JEDocSrcP_USERE_DATE  P_DATE  CurLine
2011617 GJ448 Bruce12/20/1101/01/11USD1500
2011617 GJ349áBruce12/20/1101/01/11USD1500
2011617 GJ350 Bruce12/20/1101/01/11USD1500
2011617 GJ351 Bruce12/20/1101/01/11USD1500
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First 20:  Year*JEDocSrcP_
First 20:  2011617 GJ448 Bruce1
First 20:  2011617 GJ349áBruce
First 20:  2011617 GJ350 Bruce1
First 20:  2011617 GJ351 Bruce1
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but I think it is counting octets

Note that the hexdump of your input data has a Byte Order Mark (BOM) at the front of it, which Perl counts at least as some characters.

Discounting the BOM, I get the expected output with the following program:

#!/usr/bin/perl -w
use strict;
use Encode qw/encode decode/;

open (INFILE, "<:encoding(UTF-8)", "utf8.txt") || die "blah blah blah"
+;
open (OUTFILE, ">:encoding(UTF-8)", "oututf8.txt") || die "blah blah";
binmode STDOUT, ':encoding(UTF-8)';

print "Ruler : [12345678901234567890]\n";
while (my $line = <INFILE>) {
  chomp ($line);
  print "Input : [$line]\n";
  my $linestart = substr($line,0,20);
  my $outline = $linestart;
  print "20    : [$outline]\n";
  print "---\n";
  print OUTFILE "$outline\n";
}
close (INFILE);
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To remove the BOM at the start of your file, use maybe simply

$line =~ s!^\N{BYTE ORDER MARK}!!;
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Thank you for the suggestion - but I get the same results. I think there is some system setting on the laptop that will not read in characters - it just does bytes.