If the "seq_name" line is _always_ directly after the "StrCatDone" line, then this (untested) one-liner should do it (not very memory-efficient though):

perl -0777 -pe 's/(?<="StrCatDone" "1";\s+"seq_name" ")sequence.V5_0(?
+=";)/sequence_FOO/sg' datafile.txt
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Alternatively, you could set $/='}' (see perlvar) and apply that regex to each "line", writing out to another file as you go.