use strict;
use warnings;

my $saw_an_s = 0;
while (my $current_line = <DATA>) {
    next if ($saw_an_s and $current_line =~ /^s/);
    $saw_an_s = $current_line =~ /^s/ ? 1 : 0;
    print $current_line;
}
__DATA__
s 1
s 2
s 3
a

a foo
s 4
s 5
s 6
a bar