$ cat sessions.txt
00:30:11
00:42:04
00:21:29
00:43:21
00:41:26
00:33:18
00:27:35
00:40:42
00:41:09

$ perl -F: -lane '$a+=$F[0]*3600+$F[1]*60+$F[2];END{printf "%d:%02d:%02d\n",$a/3600,$a%3600/60,$a%60}' \
    sessions.txt
5:21:15

$ awk -F: '{ a += $1*3600+$2*60+$3 } \
END { printf "%d:%02d:%02d\n",a/3600,a%3600/60,a%3600%60 }' sessions.txt
5:21:15

$ export t=0; cat sessions.txt \
| while read; do a=$(pdd -t $t $REPLY --add|head -n1); t=$a; echo $t; done \
| tail -n1
05:21:15

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