Do something like this (always use placeholders when passing variables to your SQL DB):

use strict;
use Data::Dumper;

my (@array,@newarray);

@array=("ABCDEF01","HIJKL678","VWXYZ123H");
# join 'USERNAME LIKE ?'x3 with ' OR '
my $str = join(' OR ', ('USERNAME LIKE ?')x@array);
# prints USERNAME LIKE ? OR USERNAME LIKE ? OR USERNAME LIKE ?
print $str;
my $sth = $dbh->prepare("SELECT * FROM users WHERE $str");
# use map to convert each username in @array to 'match%'
# passes (ABCD%, HIJK%, VWXY%) to execute
$sth->execute( map {  substr($_,0,4) . '%' } @array );
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