#--------1---------2---------3---------4---------5---------6---------7---------8--------
#234567890123456789012345678901234567890123456789012345678901234567890123456789012345678
perl -anF, -le'$h{$F[0]}|=$^F;$^F-=eof}{"@_"=~/[12]$/&&print"$&:$`"while(@_=each%h)' t1.csv t2.csv
1:8 
1:6 
2:1 
2:3 
2:5

##</code><code>##

#--------1---------2---------3---------4---------5---------6---------7-------
#2345678901234567890123456789012345678901234567890123456789012345678901234567
perl -pe'@h{/([^,]*)/}|=2-@ARGV}while(@_=each%h){$_="@_"=~/[12]$/&&"$&:$`$/"'