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my %hash2 = @hash{ grep /\d+_\d+/, keys %hash };

my %hash2;
@hash2{ @$_ } = @hash{ @$_ } for [ grep /\d+_\d+/, keys %hash ];
...
    my @key = grep /\d+_\d+/, keys %hash;
    @hash2{ @key } = @hash{ @key };
}