WHERE a.user_id = b.remote_id

 my %where  = (
        user_id   => 'b.remote_id'
    );

WHERE a.user_id = 'b.remote_id'

%where = (
  't.col1' => \'= t.col2',
);
my ($stmt) = $sql->where( \%where);
print $stmt,"\n";

my $column_2 = '= t.col2';

my %where = (
  't.col1' => \$column_2,
);
my ($stmt) = $sql->where( \%where);
print $stmt,"\n";

#!/usr/bin/perl

use strict;
use warnings;
use lib qw(./lib);
use SQL::Abstract;

my $sql = SQL::Abstract->new;

print "VERSION: ", SQL::Abstract->VERSION(), "\n";

my %where = (
  't.col1' => \'= t.col2',
);
my ($stmt) = $sql->where( \%where);
print $stmt,"\n";

C:\>abstract_test.pl
VERSION: 1.22
 WHERE ( t.col1 = t.col2 )