Untested.

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my @account = qw( 96329XY 7776fc 334457yg 2233oop 000001 8877666b);
my @file =  qw(8877666b.txt 2233oop.txt 334457yg.txt 7776fc.txt 96329X
+Y_K.txt);

my %seen;

for my $acc (@account) {
    next unless $acc =~ /^\d+/;
    $seen{ $acc }++; # I'm not sure why you were pushing onto an array
+ to test for existence.
}

#print Dumper %seen;

for my $filename (@file) {
    next unless $filename =~ /^(\d+.*?)\.txt$/;
    my $basename = $1;
    print "seen in account array: $basename\n" if exists $seen{ $basen
+ame }; 
}
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