I just got into the habit of sub declaration, because I remember warnings being generated if you didn't.

Its all about parens. If you use parens, no warnings. You use parens :)

$ perl -le " f(1); sub f{warn@_} "
1 at -e line 1.

$ perl -le " f 1 ; sub f{warn@_} "
Number found where operator expected at -e line 1, near "f 1"
        (Do you need to predeclare f?)
syntax error at -e line 1, near "f 1"
Execution of -e aborted due to compilation errors.

$ perl -le " sub f; f 1 ; sub f{warn@_} "
1 at -e line 1.
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Defining a sub also "declares" it:

$ perl -le "sub f{warn@_} f 1"
1 at -e line 1.
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Defining a sub also "declares" it

Except when it declares it too-late for the parser to notice

The OP uses parens, so the parser doesn't need a "forward" declaration or re-arranging of sourcecode