Hi bhargavkanakiya,
from your code: 

foreach my $filename( @files ){
    my $file = "text_000".$variable."."."lab";
    my $outfile = "$outdirname/"."$file";
    open my $output , ">$outfile" || die "Can't open the output file!"
+;
    print "file: $file\n";
    while ( my $line = <$filename> ) {
        $line =~ s/^\s+//;
        $line =~ s/ +/ /g;
        $line =~ s/\s+$/\n/g;
        print $output $line;
    }
    close ($output);
    $variable++;
}

[download]
$filename is the name of the file to open, and read from. In your code, "$filename" is not a FILEHANDLE.
In Perl Programming, you need a file-handle in the open function like thus:  open FILEHANDLE,MODE,EXPR
Note: that you are not opening, your file named "$filename" to read from. You are just using it in the while loop with readline function as while ( my $line = <$filename> ) {...} which should really be while ( my $line = <FILEHANDLE> ) {...} So, your code should be something like: 
foreach my $filename( @files ){
    my $file = "text_000".$variable."."."lab";
    my $outfile = "$outdirname/"."$file";
    open my $output,'>',$outfile or die "Can't open the output file: $
+!"; ## updated
        open my $fh,'<',$filename or die "can't open file: $!"; ## add
+ed
    print "file: $file\n";
    while ( my $line = <$fh> ) {
           $line =~ s/^\s+//;
       $line =~ s/ +/ /g;
           $line =~ s/\s+$/\n/g;
           print $output $line;
    }
        close $fh or die "can't close file: $!";  ## added
    close $output or die "can't close file: $!"; ## updated
    $variable++;
}

[download]
NOTE: All codes are not tested please.

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

In reply to Re^3: help with error 'readline() on unopened filehandle' by 2teez
in thread help with error 'readline() on unopened filehandle' by bhargavkanakiya

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