You can turn on a certain kind of warning for a block or function, like

if (do { no warnings "uninitialized"; $foo eq "true" }) { ... }
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IMO, there is nothing wrong with disabling a warning, as long as you do it only to certain warnings, not all (like no warnings;), and only for the part of code where the warning is ok. This is especially important with warnings such as no warnings "exiting"; for which there is no such easy way to avoid.

In this case however, I see nothing wrong with the explicit $foo && test, I do not think it is really bad. For a more comprehensible code, I'd write something like

if (defined($foo) && $foo eq "true") { ... }
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or

if (($foo || "") eq "true") { ... }
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or even

if ((defined($foo) ? $foo : "") eq "true") { ... }
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