This is untested, and I'm not a shell string guru, but maybe you are looking just to quiet the output of the command? Maybe try moving the /dev/null to the other side of the pipe

#untested
open(IN, "gzip -dc file.gz   2 > /dev/null  | ");
#or
open(IN, "gzip -dc file.gz   2>&1 > /dev/null  | ");
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