You've omitted the sigil when declaring $arg1 - were you not using strict? Here's the trivial example, anyway:

$ cat opts.pl 
#!/usr/bin/env perl 
use strict;
use warnings;
use Getopt::Long 'GetOptions';

my $arg1;
my %opt = ('arg1=s' => \$arg1);
GetOptions (%opt);
print "arg1 is $arg1\n" if defined $arg1;
$ ./opts.pl --arg1=foo
arg1 is foo
$
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