use strict;
use warnings;
use IO::Handle;

while (<>) {
    my $lineno = ARGV->input_line_number;
    # print a status message every 100 lines
    warn "Processing $ARGV line $lineno\n" unless $lineno % 100;
    # do some processing stuff
} continue {
    close ARGV if ARGV->eof; # else input_line_number won't reset per-file
}