@a = grep not exists $in_b{$_}, @a;
@b = grep not exists $in_a{$_}, @b;
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According to my perl:

Not enough arguments for grep at j.pl line 14, near "@a;"
Execution of j.pl aborted due to compilation errors.
[download]

I think you want:

#!/usr/bin/perl

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

my @a = (2, 6.5 ,4, 3, 2, 7.5, 4.0);
my @b = (2, 6.52 ,4.5, 7.05, 2.0, 4.1, 3);

my %in_a;
my %in_b;
undef @in_a{@a};
undef @in_b{@b};
@a = grep {not exists $in_b{$_}} @a;
@b = grep {not exists $in_a{$_}} @b;

print Dumper(\@a);
print Dumper(\@b);
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