$ cat -vet working.txt
H$
 after 1 space$
  after 2 spaces$
no leading spaces$
text^I^Ifollowed by 2 tabs$
target for^Iswap1$
...$
source for^Ireplace1$
...$
^LH$
 after 1 space$
  after 2 spaces$
no leading spaces$
text^I^Ifollowed by 2 tabs$
target for^Iswap2$
...$
source for^Ireplace2$
...$
^LH$
 after 1 space$
  after 2 spaces$
no leading spaces$
text^I^Ifollowed by 2 tabs$
target for^Iswap3$
...$
source for^Ireplace3$
...$

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;
use constant {
    CHANGE_LINE => 4,
    SOURCE_LINE => 6,
    PAGE_SEP => "\n\fH\n",
};

use File::Copy 'copy';

my $work_file = 'working.txt';
my $bu_file = 'working.txt.bu';

copy($work_file, $bu_file)
    or die "Can't 'copy($work_file, $bu_file)': $!";

{
    open my $in_fh, '<', $bu_file;
    open my $out_fh, '>', $work_file;

    local $/ = PAGE_SEP;

    while (<$in_fh>) {
        chomp;
        my @lines = split /\n/;
        my ($change_line, $source_line)
            = (CHANGE_LINE, SOURCE_LINE);
        ++$change_line, ++$source_line if $. == 1;
        my ($replace) = $lines[$source_line] =~ /(\S+)$/;
        $lines[$change_line] =~ s/\S+$/$replace/;
        print $out_fh join("\n", @lines),
                      (eof($in_fh) ? "\n" : PAGE_SEP);
    }
}

##</code><code>##

$ cat -vet working.txt
H$
 after 1 space$
  after 2 spaces$
no leading spaces$
text^I^Ifollowed by 2 tabs$
target for^Ireplace1$
...$
source for^Ireplace1$
...$
^LH$
 after 1 space$
  after 2 spaces$
no leading spaces$
text^I^Ifollowed by 2 tabs$
target for^Ireplace2$
...$
source for^Ireplace2$
...$
^LH$
 after 1 space$
  after 2 spaces$
no leading spaces$
text^I^Ifollowed by 2 tabs$
target for^Ireplace3$
...$
source for^Ireplace3$
...$

##</code><code>##

$ perl -E 'my $x = "|\t|"; say $x; say length $x;'
|       |
3

##</code><code>##

$ perl -E '
    my $x = "PROMPT\t\tTO CHANGE";
    my $len = length $x;
    my $from_index = rindex($x, "\t") + 1;
    my $from_pos = $from_index + 1;
    say $x;
    say "$from_pos through $len";
    say substr $x, $from_index, $len - $from_index;
    say substr $x, $from_index;
'
PROMPT          TO CHANGE
9 through 17
TO CHANGE
TO CHANGE