Here are a couple of ideas.

If the file you open for processing has a fully readable filehandle, you can count the lines in the filehandle, to use as a max in the Tk::Progressbar, then rewind the filehandle to process it. Increment the progressbar 1 for each line processed. Here is an example that will make a progressbar for a couple of files.

sub do_while {
use Fcntl qw(:seek);
#count lines and rewind
while(<FILE0>){$linetotal++}
  seek FILE0, 0, SEEK_SET or die "Cannot rewind file: $!";

while(<FILE1>){$linetotal++} 
  seek FILE1, 0, SEEK_SET or die "Cannot rewind file: $!";
              
while(<FILE2>){$linetotal++} 
  seek FILE2, 0, SEEK_SET or die "Cannot rewind file: $!";

print "$linetotal\n";
             
$progressbar = $main->ProgressBar(
       -length => 200,  # Actually width 
       -width  => 20,   # Actually height 
       -gap    => 0,
       -value  => 0,
       -colors => [0, 'pink'],
          )->pack(-pady => 5, -padx => 5);     

    while (<FILE0>) {
        my $line = $_;
        chomp($line);
        do_opt0($line);
        if ( $line ne $token ) {
            print SFILE "$token";
        }
    $progressbar->value($progressbar->value + (100/$linetotal) );
    $main->update;
       }

    while (<FILE1>) {
        my $line = $_;
        chomp($line);
        do_opt1($line);
        if ( $line ne $token ) {
            print SFILE "$token";
        }
    $progressbar->value($progressbar->value + (100/$linetotal) );
    $main->update;
    }

    while (<FILE2>) {
        my $line = $_;
        chomp($line);
        do_opt2($line);
        if ( $line ne $token ) {
            print SFILE "$token";
        }
    $progressbar->value($progressbar->value + (100/$linetotal) );
    $main->update;
    }
}
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If you don't know the size of the files, like if its coming in off of a network, you could use the Tk::ExecuteCommand module, which opens up a nice box, has a blinking cancel button, and shows STDOUT and STDERR. Here is a simple example:

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::ExecuteCommand;

my %commands = (
#buttontext => command 
sleep => 'sleep 20',
dir   => 'dir',
ls    => 'ls -la',
);

my $mw = MainWindow->new;

foreach my $key (keys %commands){
  $mw->Button(-text => $key,
            -command=>[\&execute, $commands{$key}] )->pack;
  }

$mw->Button(-text => 'Quit',
            -command=> sub{Tk::exit} )->pack;

MainLoop;

sub execute {
  my $command = shift;

  my $tl = $mw->Toplevel(  );
          $tl->title($command);
          $tl->Button(-text => "Close",
            -command => sub { $tl->withdraw })->pack;

   my  $ec = $tl->ExecuteCommand(
      -command    => $command,
      -entryWidth => 50,
      -height     => 10,
      -label      => '',
      -text       => 'Execute',
                    )->pack;

$ec->bell;
$ec->update;

}
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