If that's all your doing to the file, sed is tailor-made for this kind of operation (from a *nix shell prompt:)

sed -e 's/     /;/g' <YourFile.tsv >OutputFile.ssv
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(where the whitespace in the regex is meant to be a tab)

Update: This assumes consecutive tabs should be converted to the same number of consecutive spaces. Otherwise I think I would be more inclined with:

perl -e 'while( <> ){ s/(\t)+/\;/g; print $_; }' <YourFile.tsv >Output
+File.scsv
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