Your undefined value is probably later on in the elsif chain. This is a common problem.

my $def1 = 'true';
my $def2 = 'foo';
my $def3 = 'blue';
my $undef1;

if ( $def1 =~ /X/ ) {   # <-- problem reported here
    print 'not here';
}
elsif ( $def2 =~ /Y/ ) {
    print 'not here either';
}
elsif ( $undef1 =~ /$def3/ ) { # <-- problem exists here
    print 'oh noes';
}
else {
    print "happiness ensues\n";
}
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