for real , but I'm just guessing

#!/usr/bin/perl --
use strict;
use warnings;

Main( @ARGV );
exit( 0 );

sub Main {
    my( @files ) = @_;
    if( not @files ){
        @files = \q{CSEK.1.ABC19_KLJK_TT1.dat
CSEK.1.ABC19_AB_TT1.dat
Check_1.ABC19.dat.gz
CSEK.1.ABC19.dat
};
        
    }

    for my $file( @files ){
        doFileStuff( $file );
    }
}

sub doFileStuff {
    my( $file ) = @_;
    
    open my( $filehandle ), '<', $file;
    my $nfile = 0;
    
    while( my $file = readline $filehandle ){
        $file =~ s/\s+$//; ## trim trailing whitespace
        
        if ($file =~ m/^Check/)
        {
            $nfile++;
            print "File - $nfile :=$file\n";
        }
        elsif ($file =~ m/^CSEK.*TT1.dat$/)
        {
            $nfile++;
            print "File - $nfile :=$file\n";
        }
        elsif ($file =~ m/^CSEK.dat$/)
        {
            $nfile++;
            print "File - $nfile :=$file\n";
        }
        else
        {
            printf "%10s %s\n", ' ',  $file;
        }
    }
    close $filehandle;
}
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