my $str = "This is my program";
my @words = split ' ', $str;

##</code><code>##

my @chunks = split /(\s+)/, $str;

##</code><code>##

shift @chunks if $chunks[0] eq '';  # and then
my $s = $chunks[0] =~ /\s/ ? 1 : 0;
my $e = $chunks[-1] =~ /\s/ ? -2 : -1;

##</code><code>##

my ($s,$e)=map $_ + ($chunks[$_] =~ /\s/ ?
    ($_ >=0 ? 1 : -1 ) : 0) => 0, -1;

##</code><code>##

map {s/[^a-z ]//gi} @words;

##</code><code>##

@words = map { (my $s=$_) =~ s/[^a-z ]//gi; $s } @words;

##</code><code>##

y/\0//d for @chunks;  # or @chunks[$e,$s];

##</code><code>##

for ([split /(\s+)/, $str]) {
    shift @$_ if $_->[0] eq '';
    my ($s,$e)= do {
        my @c=@$_;
        map $_ + ($c[$_] =~ /\s/ ?
            ($_ >=0 ? 1 : -1 ) : 0) => 0, -1;
    };
    $_->[$s] ^= $_->[$e];
    $_->[$e] ^= $_->[$s];
    $_->[$s] ^= $_->[$e];
    y/\0//d for @$_;
    local $"='';
    print "@$_";
}