You hadn't specified the arguments were regex patterns, not text.

perl -le'while(<STDIN>) { print $1 if /^$ARGV[0]:([^:]*):$ARGV[1]:/ }'
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If they are actually text, then you can easily solve that for the Perl version.

perl -le'while(<STDIN>) { print $1 if /^\Q$ARGV[0]\E:([^:]*):\Q$ARGV[1
+]\E:/ }'
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Using split is bit longer, but more readable.