# in the code
my $arg = hex(shift @ARGV);
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Wouldn't that rather be

my $arg = chr hex(shift @ARGV);
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(hex gives a number (e.g. 209 for "d1"), but it seems the OP actually wants the respective char, i.e. chr(209)... )

BTW, another possibility would be

my $sep = pack "H2", (shift @ARGV);  # with input being 'd1', for exam
+ple
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Or, if you insist on specifying the \x prefix on input, you could extract the hex part with a regex

my $sepArg = shift @ARGV;
my ($hex) = $sepArg =~ /\\x([\da-fA-F]+)/;
my $sep = chr hex $hex;
my $str = "field1 $sep field2";
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Update: in theory, you could also do

my $sepArg = shift @ARGV;            # input being '\xd1', for example
my $str = eval qq("field1 $sepArg field2");
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but don't do that if your user input comes from untrusted sources... (any code could be executed!)