You might want to take a look at Data::Phrasebook, as this handles things a little better. Using your example the following works:

use Data::Phrasebook;
my $pb = Data::Phrasebook->new( file => 'phrases.txt' );
my $str = $pb->fetch( 'baz', {foo => '${bar}', bar => '${foo}'} );
# $str = 'foo is ${bar} and bar is ${foo}'
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where phrases.txt use the default parameter substitution and looks like:

baz=foo is :foo and bar is :bar
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While there is support for TT style parameter substitution, it doesn't current support a TT embeded templating system. Might be a possibility for the future, but would be a little overkill for this problem ;)