Use the can method (provided by the UNIVERSAL base class)...

my $method = "build_a_kite";
if ($obj->can($method)) {
    print "We can!";
}
else {
    print "We can't...";
}
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From perldoc UNIVERSAL...

$obj->can( METHOD ), CLASS->can( METHOD ), can( VAL, METHOD )

can checks if the object or class has a method called METHOD. If
it does then a reference to the sub is returned. If it does not
then undef is returned. This includes methods inherited or
imported by $obj, CLASS, or VAL.

can cannot know whether an object will be able to provide a method
through AUTOLOAD, so a return value of undef does not necessarily
mean the object will not be able to handle the method call. To get
around this some module authors use a forward declaration (see
perlsub) for methods they will handle via AUTOLOAD. For such
'dummy' subs, can will still return a code reference, which, when
called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD
is provided, calling the coderef will cause an error.

can can be called as a class (static) method, an object method, or
a function.

When used as a function, if VAL is a blessed reference or package
name which has a method called METHOD, can returns a reference to
the subroutine. If VAL is not a blessed reference, or if it does
not have a method METHOD, undef is returned.
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