my $str = <<'EOB';
start
    hehu7 gjwsdh hsjdhj remove hjhsj
    jdkwj hwjdhjwh
end
start
    crgr hjdhwjd
    wjdkwj wkjw g hjhd
end
EOB

while ($str =~ /(\bstart\b.*?\bend\b\s*)/gs) {
    my $block = $1;
    if ($block =~ /\bremove\b/) {
        $str =~ s/\Q$block\E//;
    }
}
print $str;

__END__
start
    crgr hjdhwjd
    wjdkwj wkjw g hjhd
end