No, that won't work. The use inside the eval BLOCK is compiled at compile time, but the string in eval EXPR is not. eval "use Foo"; is nothing but an obfuscated way to say require Foo; Foo->import;

Neither case is a job for eval here. If you want variable paths at compile time, you do the following:

my $libpath;
BEGIN { $libpath = "/foo/bar"; }
use lib $libpath;
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If you want to load modules with dynamic @INC at runtime, you make use of the aforemention construct:

{
    local @INC = ("/foo/bar", @INC);
    require Foo;
    Foo->import;
}
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eval will not help. And $a is a really awful name for a variable (plus $a and $b are reserved for sort). :)

Makeshifts last the longest.