In that case, dup(2)ing the file handle is probably a better idea:

open IN, "<&SOCKET" or die ...;
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though selecting a suitable non-bareword file handle scheme is even better.

my $in = ...;
open $in, "<&".fileno($sock) or die ...;
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Where the first "..." depends on what you find appropriate. "undef" works for modern versions of Perl. Or you can use IO::Handle or other module or just do { local(*IN); \*IN }.