Update:I cleaned up a redundant subroutine call in my code.

You need to get the information out with a custom sql statement and put it in some kind of data structure if it exists. There are a lot of ways to do this. a quick example.

#!/usr/bin/perl -w

use strict;
use DBI;
#db connect
my $driver = 'mysql';
my $database = 'usersdb';
my $dsn = "DBI:$driver:database=$database";
my $dbh = DBI->connect($dsn, undef, undef);
my $user ="bob";
my $user_test = user_exists($user);
if (defined $user_test){
    print "user $user is already in our database";
}else{
    #code to insert user
}

$dbh->disconnect;

sub user_exists{
    my $id_val = shift;
    my $statement = "select user_id from users where user_id = $id_val
+";
    my $ary_ref = $dbh->selectall_arrayref($statement);
    if (defined $ary_ref->[0]->[0]){
        return $ary_ref->[0]->[0];
    }else{
        return undef;
    }
    
}
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