That is what regular expressions are for:

local $_ = <STDIN>; chomp;
my $hexnum = undef;
$hexnum = eval "0x$1" if (m/^(?:0[xX])?([[:xdigit:]]+)$/);
die "bad input" unless defined $hexnum;
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Do not assume that the input meets your constraints — verify that the input meets your constraints! :-)