If the warning isn't clear enough (even though it tells you exactly what to do in this case), you can use diagnostics to get more info.

>perl -Mdiagnostics -e"my $x; if ($x = 2) {}"
Found = in conditional, should be == at -e line 1 (#1)
    (W syntax) You said

        if ($foo = 123)

    when you meant

        if ($foo == 123)

    (or something like that).
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Ok, so it doesn't add much in this case.