my $filename = 'somefile';
my $rec_len  = 567;

open my $IN,  '<:raw',  $filename or die "Cannot open '$filename' $!";
open my $OUT, '+<:raw', $filename or die "Cannot open '$filename' $!";

my $file_size     = -s $IN;
my $total_records = $file_size / $rec_len;

$/ = \$rec_len;
while ( <$IN> ) {
    next if $. == 1;
    print $OUT $_;
    }

$total_records == $. or die "Read only $. records but expected $total_records records!\n";

truncate $IN, $file_size - $rec_len;

close $OUT;
close $IN;