my @m_h = grep {local($_);s/tw/tr/g; /^t.*/ig;$_} @foo;

##</code><code>##

my @m_h = map {s/tw/tr/g;$_} @foo;

##</code><code>##

my @m_h = @foo;
s/tw/tr/g for @m_h;

##</code><code>##

my @m_h = grep /^t/i, @foo;
s/tw/tr/g for @m_h;