It may be simpler to just localize *STDIN,

{
    open local(*STDIN), '<', "whatever" or die $!;
    # go on with processing,
    # calls to any sub will think STDIN is
    # the one you just set up.
    # ...
    close STDIN or die $!;
}
# old STDIN is restored, never been touched.
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Explicit duplication is sometimes necessary, but not usually.

After Compline,
Zaxo