#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %hashA = ( '11.5'  => [ 1723478, 1734789, 1798761, ],
              '11.12' => [ 1700123, ],
              '11.01' => [ 1780345, ]        
         );

print Dumper (\%hashA);

$hashA{q/11.5/} = [ grep { $_ != 1734789 } @{$hashA{q/11.5/}} ];

print Dumper (\%hashA);

__END__
Prints:
$VAR1 = {
          '11.01' => [
                       1780345
                     ],
          '11.12' => [
                       1700123
                     ],
          '11.5' => [
                      1723478,
                      1734789,
                      1798761
                    ]
        };
$VAR1 = {
          '11.01' => [
                       1780345
                     ],
          '11.12' => [
                       1700123
                     ],
          '11.5' => [
                      1723478,
                      1798761
                    ]
        };