Anytime you substitute inside of a grep or map block, you have to do this: 
my @m_h = grep {local($_);s/tw/tr/g; /^t.*/ig;$_} @foo;

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That is, put the variable you are substituted as the last item in the block. Otherwise, you return the return of the substitute, which is usually 1 or the number of matches (i always forget :/).

In your case, you don't want grep - you want map (actually, you do want grep ... see 2nd update below): 

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

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Notice that you don't need local.

UPDATE:
Oops, this still changes the contents of @foo ... hmmm, how about a simple copy? 

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

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Should be all you need. No?

UPDATE 2:
After looking at ctilmes reply, how about this: 

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

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Hope this helps. :)

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to Re: Localize $_ in grep by jeffa
in thread Localize $_ in grep by InfiniteSilence

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