$ cat pm_1077685_A.txt
Line 1: no plate info
Line 2: ... PlateID 123 ...
Line 3: ... PlateID 456 ...
Line 4: no plate info

$ cat pm_1077685_B.txt
Line 1: no plate info
Line 2: ... PlateID 789 ...
Line 3: ... PlateID ...
Line 4: no plate info

$ cat pm_1077685_C.txt
Line 1: no plate info
Line 2: ... PlateID 000 ...
Line 4: no plate info

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

my @filenames = qw{pm_1077685_A.txt pm_1077685_B.txt pm_1077685_C.txt};

for my $filename (@filenames) {
    my $in_path = "./$filename";
    print "Processing: '$in_path'\n";
    open my $in_fh, '<', $in_path;

    while (<$in_fh>) {
        print "\t$_";
        if (/PlateID/) {
            if (/PlateID (\d+)/) {
                my $plate_id = $1;
                print "\t\tProcess PlateID '$plate_id' from '$in_path'\n";
            }
            else {
                print "\t\tpopup_error_window($in_path)\n";
                last;
            }
        }
    }
}

##</code><code>##

$ pm_example.pl
Processing: './pm_1077685_A.txt'
	Line 1: no plate info
	Line 2: ... PlateID 123 ...
		Process PlateID '123' from './pm_1077685_A.txt'
	Line 3: ... PlateID 456 ...
		Process PlateID '456' from './pm_1077685_A.txt'
	Line 4: no plate info
Processing: './pm_1077685_B.txt'
	Line 1: no plate info
	Line 2: ... PlateID 789 ...
		Process PlateID '789' from './pm_1077685_B.txt'
	Line 3: ... PlateID ...
		popup_error_window(./pm_1077685_B.txt)
Processing: './pm_1077685_C.txt'
	Line 1: no plate info
	Line 2: ... PlateID 000 ...
		Process PlateID '000' from './pm_1077685_C.txt'
	Line 4: no plate info