You could use the AUTOLOAD approach, or, if you know ahead of time which functions might cause trouble, you can use an eval BLOCK approach:

# I know foo() and bar() exist...
# but not too sure of xyzzy() or quux()

foo();

eval { xyzzy() };
# $@ will have a value if xyzzy() isn't defined

bar();

eval { quux() };
# $@ will have a value if quux() isn't defined
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