There's not a built-in function, but it's trivial to write one. Just use the symbol table:

#!/usr/bin/perl -w

use strict;

package Parent;

sub new {
    bless({}, $_[0]);
}

package Child;

@Child::ISA = qw( Parent );

sub child_only {}

package main;

print "new exists in Child\n" if in_kid('Child', 'new');
print "new exists in Parent\n" if in_kid('Parent', 'new');
print "child_only exists in Child\n" if in_kid('Child', 'child_only');
print "child_only exists in Parent\n" if in_kid('child_only', 'new');

sub in_kid {
    my $class = shift;
    my $sub = shift;
    no strict 'refs';
    return defined &{$class . '::' . $sub};
}
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