Thanks to Andy Lester:

#! /bin/sh

# Process pasteboard contents to md table. Thanks, Andy Lester.
# Copy table from Numbers or other tab-delimited spreadsheet.
pbpaste | perl -le'$_=<>;&x;s/[^|]/-/g;&x;for(<>){&x}sub x{chomp;s/\t/|/g;print"|$_|";}' | pbcopy

# Done.
echo "Styled pasteboard to table"