as the wise Corion already said in list context (given by the array @lines = < ) the diamond operator read all the file and so you r next call, in scalar context this time $line=<FILE1> , to the diamond operator will retrieve nothing.

I can just add to always use use strict; use warnings at the very beginning of your program.

Also I suggest you to always use the 3 args form for open and using a lexical scoped filehadle instead of the bareword one:

open my $file_handle, "<", $filepath or die "unable to open [$filepath] in reading monde!";

also the autodie can be useful for such open situation.

L*

PS: it seems i'm slow answering this morning..