#!/usr/bin/perl -w
use strict;
my @avoid= map qr/\Q$_\E/i, qw(leroy brown);
my $log= "/home/psmith/logfile";
open( IN, "< $log" )  or  die "Can't read $log: $!\n";
my $line;
while( $line= <IN> ) {
    print $line   unless  grep $line =~ $_, @avoid;
}
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If @avoid is quite large, then having grep always match against all of them might be worth avoiding. Maybe one day grep will be optimized for this case. (: Until then:

while( $line= <IN> ) {
    for(  @avoid  ) {
        if(  $line =~ $_  ) {
            print $line;
            last;
        }
    }
}
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while( $line= <IN> ) {
    for(  @avoid  ) {
        if(  $line =~ $_  ) {
            $line= "";
            last;
        }
    }
    print $line;
}
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Your second code prints if in @avoid. I hope it's Ironic(TM).