[TxMt] Improved Perl "Read File" snippet

Hi. I happened to notice that the Perl "Read File" snippet was using bareword filehandles and doing a lot more work than it has to. The following uses lexical filehandles, 3-argument open (to prevent anything in the filename from being misinterpreted as an open sigil) and a do block to simplify the assignment.

Finally, a third, blank position is provided so you can tab straight to the next line instead of the end of the block.

my $${1:var} = do { local $/; open my $fh, "<", "${2:file}"; <$fh>; }; $3

PS It's worth mentioning that this is backwards compatible to 5.6 which should be more than plenty these days.