in reply to search for a string and return previous relating string if found

Try this: 
perl -nE "m/^interface (\S+)/ and $int=$1; m/ spanning-tree guard root
+/ and print qq|$int $_|" YOUR-FILE.txt

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prints: 
FastEthernet1/0/3  spanning-tree guard root

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Somewhat simple-minded, but should do the trick.

                All power corrupts, but we need electricity.

In Section Seekers of Perl Wisdom